Index: lib/Transforms/InstCombine/InstCombineCasts.cpp
===================================================================
--- lib/Transforms/InstCombine/InstCombineCasts.cpp
+++ lib/Transforms/InstCombine/InstCombineCasts.cpp
@@ -1053,11 +1053,17 @@
   if (CI.hasOneUse() && isa<TruncInst>(CI.user_back()))
     return nullptr;
 
+  Value *Src = CI.getOperand(0);
+
+  // If this zero extend is of phi, keep it that way.
+  // This may allow further simplifications.
+  if (CI.hasOneUse() && isa<PHINode>(Src))
+    return nullptr;
+
   // If one of the common conversion will work, do it.
   if (Instruction *Result = commonCastTransforms(CI))
     return Result;
 
-  Value *Src = CI.getOperand(0);
   Type *SrcTy = Src->getType(), *DestTy = CI.getType();
 
   // Attempt to extend the entire input expression tree to the destination
@@ -1324,10 +1330,16 @@
   if (CI.hasOneUse() && isa<TruncInst>(CI.user_back()))
     return nullptr;
 
+  Value *Src = CI.getOperand(0);
+
+  // If this zero extend is of phi, keep it that way.
+  // This may allow further simplifications.
+  if (CI.hasOneUse() && isa<PHINode>(Src))
+    return nullptr;
+
   if (Instruction *I = commonCastTransforms(CI))
     return I;
 
-  Value *Src = CI.getOperand(0);
   Type *SrcTy = Src->getType(), *DestTy = CI.getType();
 
   // If we know that the value being extended is positive, we can use a zext
Index: test/Transforms/InstCombine/ext-of-select-of-bittest.ll
===================================================================
--- /dev/null
+++ test/Transforms/InstCombine/ext-of-select-of-bittest.ll
@@ -0,0 +1,130 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt < %s -instcombine -S | FileCheck %s
+
+; https://bugs.llvm.org/show_bug.cgi?id=17564
+
+; If we thread zext/sext into phi, we won't be able to get rid of select
+; later on, so don't do that.
+
+define zeroext i1 @just_phi(i32) {
+; CHECK-LABEL: @just_phi(
+; CHECK-NEXT:    [[TMP2:%.*]] = and i32 [[TMP0:%.*]], 1
+; CHECK-NEXT:    [[TMP3:%.*]] = icmp eq i32 [[TMP2]], 0
+; CHECK-NEXT:    br i1 [[TMP3]], label [[TMP4:%.*]], label [[TMP7:%.*]]
+; CHECK:         [[TMP5:%.*]] = and i32 [[TMP0]], 4
+; CHECK-NEXT:    [[TMP6:%.*]] = icmp ne i32 [[TMP5]], 0
+; CHECK-NEXT:    br label [[TMP7]]
+; CHECK:         [[TMP8:%.*]] = phi i1 [ true, [[TMP1:%.*]] ], [ [[TMP6]], [[TMP4]] ]
+; CHECK-NEXT:    ret i1 [[TMP8]]
+;
+  %2 = and i32 %0, 1
+  %3 = icmp ne i32 %2, 0
+  br i1 %3, label %7, label %4
+
+  %5 = and i32 %0, 4
+  %6 = icmp ne i32 %5, 0
+  br label %7
+
+  %8 = phi i1 [ true, %1 ], [ %6, %4 ]
+  ret i1 %8
+}
+
+define i32 @zext_of_phi(i32) {
+; CHECK-LABEL: @zext_of_phi(
+; CHECK-NEXT:    [[TMP2:%.*]] = and i32 [[TMP0:%.*]], 1
+; CHECK-NEXT:    [[TMP3:%.*]] = icmp eq i32 [[TMP2]], 0
+; CHECK-NEXT:    br i1 [[TMP3]], label [[TMP4:%.*]], label [[TMP7:%.*]]
+; CHECK:         [[TMP5:%.*]] = and i32 [[TMP0]], 4
+; CHECK-NEXT:    [[TMP6:%.*]] = icmp ne i32 [[TMP5]], 0
+; CHECK-NEXT:    br label [[TMP7]]
+; CHECK:         [[TMP8:%.*]] = phi i1 [ true, [[TMP1:%.*]] ], [ [[TMP6]], [[TMP4]] ]
+; CHECK-NEXT:    [[TMP9:%.*]] = zext i1 [[TMP8]] to i32
+; CHECK-NEXT:    ret i32 [[TMP9]]
+;
+  %2 = and i32 %0, 1
+  %3 = icmp ne i32 %2, 0
+  br i1 %3, label %7, label %4
+
+  %5 = and i32 %0, 4
+  %6 = icmp ne i32 %5, 0
+  br label %7
+
+  %8 = phi i1 [ true, %1 ], [ %6, %4 ]
+  %9 = zext i1 %8 to i32
+  ret i32 %9
+}
+
+define i32 @sext_of_phi(i32) {
+; CHECK-LABEL: @sext_of_phi(
+; CHECK-NEXT:    [[TMP2:%.*]] = and i32 [[TMP0:%.*]], 1
+; CHECK-NEXT:    [[TMP3:%.*]] = icmp eq i32 [[TMP2]], 0
+; CHECK-NEXT:    br i1 [[TMP3]], label [[TMP4:%.*]], label [[TMP7:%.*]]
+; CHECK:         [[TMP5:%.*]] = and i32 [[TMP0]], 4
+; CHECK-NEXT:    [[TMP6:%.*]] = icmp ne i32 [[TMP5]], 0
+; CHECK-NEXT:    br label [[TMP7]]
+; CHECK:         [[TMP8:%.*]] = phi i1 [ true, [[TMP1:%.*]] ], [ [[TMP6]], [[TMP4]] ]
+; CHECK-NEXT:    [[TMP9:%.*]] = sext i1 [[TMP8]] to i32
+; CHECK-NEXT:    ret i32 [[TMP9]]
+;
+  %2 = and i32 %0, 1
+  %3 = icmp ne i32 %2, 0
+  br i1 %3, label %7, label %4
+
+  %5 = and i32 %0, 4
+  %6 = icmp ne i32 %5, 0
+  br label %7
+
+  %8 = phi i1 [ true, %1 ], [ %6, %4 ]
+  %9 = sext i1 %8 to i32
+  ret i32 %9
+}
+
+; After instcombine those ^ are transformed into these \/,
+; which are then simplified into icmp.
+
+define zeroext i1 @just_select(i32) {
+; CHECK-LABEL: @just_select(
+; CHECK-NEXT:    [[TMP2:%.*]] = and i32 [[TMP0:%.*]], 5
+; CHECK-NEXT:    [[TMP3:%.*]] = icmp ne i32 [[TMP2]], 0
+; CHECK-NEXT:    ret i1 [[TMP3]]
+;
+  %2 = and i32 %0, 1
+  %3 = icmp eq i32 %2, 0
+  %4 = and i32 %0, 4
+  %5 = icmp ne i32 %4, 0
+  %6 = select i1 %3, i1 %5, i1 true
+  ret i1 %6
+}
+
+define i32 @zext_of_select(i32) {
+; CHECK-LABEL: @zext_of_select(
+; CHECK-NEXT:    [[TMP2:%.*]] = and i32 [[TMP0:%.*]], 5
+; CHECK-NEXT:    [[TMP3:%.*]] = icmp ne i32 [[TMP2]], 0
+; CHECK-NEXT:    [[TMP4:%.*]] = zext i1 [[TMP3]] to i32
+; CHECK-NEXT:    ret i32 [[TMP4]]
+;
+  %2 = and i32 %0, 1
+  %3 = icmp eq i32 %2, 0
+  %4 = and i32 %0, 4
+  %5 = icmp ne i32 %4, 0
+  %6 = select i1 %3, i1 %5, i1 true
+  %7 = zext i1 %6 to i32
+  ret i32 %7
+}
+
+
+define i32 @sext_of_select(i32) {
+; CHECK-LABEL: @sext_of_select(
+; CHECK-NEXT:    [[TMP2:%.*]] = and i32 [[TMP0:%.*]], 5
+; CHECK-NEXT:    [[TMP3:%.*]] = icmp ne i32 [[TMP2]], 0
+; CHECK-NEXT:    [[TMP4:%.*]] = sext i1 [[TMP3]] to i32
+; CHECK-NEXT:    ret i32 [[TMP4]]
+;
+  %2 = and i32 %0, 1
+  %3 = icmp eq i32 %2, 0
+  %4 = and i32 %0, 4
+  %5 = icmp ne i32 %4, 0
+  %6 = select i1 %3, i1 %5, i1 true
+  %7 = sext i1 %6 to i32
+  ret i32 %7
+}
Index: test/Transforms/InstCombine/icmp-div-constant.ll
===================================================================
--- test/Transforms/InstCombine/icmp-div-constant.ll
+++ test/Transforms/InstCombine/icmp-div-constant.ll
@@ -9,15 +9,15 @@
 define i32 @icmp_div(i16 %a, i16 %c) {
 ; CHECK-LABEL: @icmp_div(
 ; CHECK-NEXT:  entry:
-; CHECK-NEXT:    [[TOBOOL:%.*]] = icmp eq i16 %a, 0
-; CHECK-NEXT:    br i1 [[TOBOOL]], label %then, label %exit
+; CHECK-NEXT:    [[TOBOOL:%.*]] = icmp eq i16 [[A:%.*]], 0
+; CHECK-NEXT:    br i1 [[TOBOOL]], label [[THEN:%.*]], label [[EXIT:%.*]]
 ; CHECK:       then:
-; CHECK-NEXT:    [[NOT_CMP:%.*]] = icmp eq i16 %c, 0
-; CHECK-NEXT:    [[PHITMP1:%.*]] = sext i1 [[NOT_CMP]] to i32
-; CHECK-NEXT:    br label %exit
+; CHECK-NEXT:    [[CMP:%.*]] = icmp eq i16 [[C:%.*]], 0
+; CHECK-NEXT:    br label [[EXIT]]
 ; CHECK:       exit:
-; CHECK-NEXT:    [[PHI:%.*]] = phi i32 [ -1, %entry ], [ [[PHITMP1]], %then ]
-; CHECK-NEXT:    ret i32 [[PHI]]
+; CHECK-NEXT:    [[PHI:%.*]] = phi i1 [ true, [[ENTRY:%.*]] ], [ [[CMP]], [[THEN]] ]
+; CHECK-NEXT:    [[ADD:%.*]] = sext i1 [[PHI]] to i32
+; CHECK-NEXT:    ret i32 [[ADD]]
 ;
 entry:
   %tobool = icmp eq i16 %a, 0
@@ -38,13 +38,12 @@
 define i32 @icmp_div2(i16 %a, i16 %c) {
 ; CHECK-LABEL: @icmp_div2(
 ; CHECK-NEXT:  entry:
-; CHECK-NEXT:    [[TOBOOL:%.*]] = icmp eq i16 %a, 0
-; CHECK-NEXT:    br i1 [[TOBOOL]], label %then, label %exit
+; CHECK-NEXT:    [[TOBOOL:%.*]] = icmp eq i16 [[A:%.*]], 0
+; CHECK-NEXT:    br i1 [[TOBOOL]], label [[THEN:%.*]], label [[EXIT:%.*]]
 ; CHECK:       then:
-; CHECK-NEXT:    br label %exit
+; CHECK-NEXT:    br label [[EXIT]]
 ; CHECK:       exit:
-; CHECK-NEXT:    [[PHI:%.*]] = phi i32 [ -1, %entry ], [ 0, %then ]
-; CHECK-NEXT:    ret i32 [[PHI]]
+; CHECK-NEXT:    ret i32 -1
 ;
 entry:
   %tobool = icmp eq i16 %a, 0
@@ -65,15 +64,15 @@
 define i32 @icmp_div3(i16 %a, i16 %c) {
 ; CHECK-LABEL: @icmp_div3(
 ; CHECK-NEXT:  entry:
-; CHECK-NEXT:    [[TOBOOL:%.*]] = icmp eq i16 %a, 0
-; CHECK-NEXT:    br i1 [[TOBOOL]], label %then, label %exit
+; CHECK-NEXT:    [[TOBOOL:%.*]] = icmp eq i16 [[A:%.*]], 0
+; CHECK-NEXT:    br i1 [[TOBOOL]], label [[THEN:%.*]], label [[EXIT:%.*]]
 ; CHECK:       then:
-; CHECK-NEXT:    [[NOT_CMP:%.*]] = icmp eq i16 %c, 0
-; CHECK-NEXT:    [[PHITMP1:%.*]] = sext i1 [[NOT_CMP]] to i32
-; CHECK-NEXT:    br label %exit
+; CHECK-NEXT:    [[CMP:%.*]] = icmp eq i16 [[C:%.*]], 0
+; CHECK-NEXT:    br label [[EXIT]]
 ; CHECK:       exit:
-; CHECK-NEXT:    [[PHI:%.*]] = phi i32 [ -1, %entry ], [ [[PHITMP1]], %then ]
-; CHECK-NEXT:    ret i32 [[PHI]]
+; CHECK-NEXT:    [[PHI:%.*]] = phi i1 [ true, [[ENTRY:%.*]] ], [ [[CMP]], [[THEN]] ]
+; CHECK-NEXT:    [[ADD:%.*]] = sext i1 [[PHI]] to i32
+; CHECK-NEXT:    ret i32 [[ADD]]
 ;
 entry:
   %tobool = icmp eq i16 %a, 0
Index: test/Transforms/InstCombine/icmp-mul-zext.ll
===================================================================
--- test/Transforms/InstCombine/icmp-mul-zext.ll
+++ test/Transforms/InstCombine/icmp-mul-zext.ll
@@ -18,11 +18,11 @@
 ; CHECK-NEXT:    [[AND:%.*]] = and i64 [[MUL3]], [[TMP2]]
 ; CHECK-NEXT:    [[CONV4:%.*]] = trunc i64 [[AND]] to i32
 ; CHECK-NEXT:    [[TOBOOL7:%.*]] = icmp eq i32 [[CONV4]], 0
-; CHECK-NEXT:    [[PHITMP:%.*]] = zext i1 [[TOBOOL7]] to i32
 ; CHECK-NEXT:    br label [[LOR_END]]
 ; CHECK:       lor.end:
-; CHECK-NEXT:    [[TMP3:%.*]] = phi i32 [ 1, [[ENTRY:%.*]] ], [ [[PHITMP]], [[LOR_RHS]] ]
-; CHECK-NEXT:    ret i32 [[TMP3]]
+; CHECK-NEXT:    [[TMP3:%.*]] = phi i1 [ true, [[ENTRY:%.*]] ], [ [[TOBOOL7]], [[LOR_RHS]] ]
+; CHECK-NEXT:    [[CONV8:%.*]] = zext i1 [[TMP3]] to i32
+; CHECK-NEXT:    ret i32 [[CONV8]]
 ;
 entry:
   %conv = zext i32 %0 to i64