Index: llvm/lib/Transforms/InstCombine/InstCombinePHI.cpp
===================================================================
--- llvm/lib/Transforms/InstCombine/InstCombinePHI.cpp
+++ llvm/lib/Transforms/InstCombine/InstCombinePHI.cpp
@@ -1300,6 +1300,12 @@
   if (Instruction *Result = foldPHIArgZextsIntoPHI(PN))
     return Result;
 
+  // Replace values from unreachable incoming blocks with undef.
+  for (unsigned i = 0, e = PN.getNumIncomingValues(); i != e; ++i) {
+    if (!DT.isReachableFromEntry(PN.getIncomingBlock(i)))
+      PN.setIncomingValue(i, UndefValue::get(PN.getType()));
+  }
+
   // If all PHI operands are the same operation, pull them through the PHI,
   // reducing code size.
   if (isa<Instruction>(PN.getIncomingValue(0)) &&
Index: llvm/test/Transforms/InstCombine/phi.ll
===================================================================
--- llvm/test/Transforms/InstCombine/phi.ll
+++ llvm/test/Transforms/InstCombine/phi.ll
@@ -1181,3 +1181,39 @@
   %cmp1 = icmp ne i32 %a.0, 0
   ret i1  %cmp1
 }
+
+; This would crash trying to delete an instruction (conv)
+; that still had uses because the user (the phi) was not
+; updated to remove a use from an unreachable block (g.exit).
+
+define void @main(i16 %x) {
+; CHECK-LABEL: @main(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    br label [[FOR_COND:%.*]]
+; CHECK:       for.cond:
+; CHECK-NEXT:    br i1 undef, label [[FOR_END:%.*]], label [[FOR_BODY:%.*]]
+; CHECK:       for.body:
+; CHECK-NEXT:    unreachable
+; CHECK:       g.exit:
+; CHECK-NEXT:    br label [[FOR_COND]]
+; CHECK:       for.end:
+; CHECK-NEXT:    ret void
+;
+entry:
+  br label %for.cond
+
+for.cond:
+  %p = phi double [ %conv, %g.exit ], [ undef, %entry ]
+  br i1 undef, label %for.end, label %for.body
+
+for.body:
+  %conv = sitofp i16 %x to double
+  unreachable
+
+g.exit:
+  br label %for.cond
+
+for.end:
+  store double %p, double* undef
+  ret void
+}
Index: llvm/test/Transforms/InstCombine/zext-or-icmp.ll
===================================================================
--- llvm/test/Transforms/InstCombine/zext-or-icmp.ll
+++ llvm/test/Transforms/InstCombine/zext-or-icmp.ll
@@ -31,14 +31,7 @@
 ; CHECK:       block1:
 ; CHECK-NEXT:    br label [[BLOCK2]]
 ; CHECK:       block2:
-; CHECK-NEXT:    [[CMP_I:%.*]] = phi i1 [ false, [[BLOCK1:%.*]] ], [ true, [[ENTRY:%.*]] ]
-; CHECK-NEXT:    [[M_011:%.*]] = phi i32 [ 0, [[BLOCK1]] ], [ 33, [[ENTRY]] ]
-; CHECK-NEXT:    [[M_1_OP:%.*]] = lshr i32 1, [[M_011]]
-; CHECK-NEXT:    [[SEXT_MASK:%.*]] = and i32 [[M_1_OP]], 65535
-; CHECK-NEXT:    [[CMP115:%.*]] = icmp ne i32 [[SEXT_MASK]], 0
-; CHECK-NEXT:    [[CMP1:%.*]] = or i1 [[CMP_I]], [[CMP115]]
-; CHECK-NEXT:    [[CONV2:%.*]] = zext i1 [[CMP1]] to i32
-; CHECK-NEXT:    ret i32 [[CONV2]]
+; CHECK-NEXT:    ret i32 1
 ;
 entry:
   br label %block2