Index: llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
===================================================================
--- llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -1452,6 +1452,27 @@
     if (Instruction *Res = processUGT_ADDCST_ADD(Cmp, A, B, CI2, CI, *this))
       return Res;
 
+  // icmp(phi(C1, C2, ...), C) -> phi(icmp(C1, C), icmp(C2, C), ...).
+  Constant *C = dyn_cast<Constant>(Op1);
+  if (!C)
+    return nullptr;
+
+  if (auto *Phi = dyn_cast<PHINode>(Op0))
+    if (all_of(Phi->operands(), [](Value *V) { return isa<Constant>(V); })) {
+      Type *Ty = Cmp.getType();
+      Builder.SetInsertPoint(Phi);
+      PHINode *NewPhi =
+          Builder.CreatePHI(Ty, Phi->getNumOperands());
+      for (BasicBlock *Predecessor : predecessors(Phi->getParent())) {
+        auto *Input =
+            cast<Constant>(Phi->getIncomingValueForBlock(Predecessor));
+        auto *BoolInput = ConstantExpr::getCompare(Pred, Input, C);
+        NewPhi->addIncoming(BoolInput, Predecessor);
+      }
+      NewPhi->takeName(&Cmp);
+      return replaceInstUsesWith(Cmp, NewPhi);
+    }
+
   return nullptr;
 }
 
Index: llvm/test/Transforms/InstCombine/icmp-constant-phi.ll
===================================================================
--- llvm/test/Transforms/InstCombine/icmp-constant-phi.ll
+++ llvm/test/Transforms/InstCombine/icmp-constant-phi.ll
@@ -2,8 +2,6 @@
 ; RUN: opt < %s -instcombine -S | FileCheck %s
 ; RUN: opt < %s -passes=instcombine -S | FileCheck %s
 
-; TODO: Replace with boolean Phi.
-
 define i1 @test_eq(i1 %cond) {
 ; CHECK-LABEL: @test_eq(
 ; CHECK-NEXT:  entry:
@@ -13,10 +11,9 @@
 ; CHECK:       if.false:
 ; CHECK-NEXT:    br label [[MERGE]]
 ; CHECK:       merge:
-; CHECK-NEXT:    [[PHI:%.*]] = phi i32 [ 123, [[IF_TRUE]] ], [ 456, [[IF_FALSE]] ]
+; CHECK-NEXT:    [[COMPARE:%.*]] = phi i1 [ true, [[IF_FALSE]] ], [ false, [[IF_TRUE]] ]
 ; CHECK-NEXT:    br label [[EXIT:%.*]]
 ; CHECK:       exit:
-; CHECK-NEXT:    [[COMPARE:%.*]] = icmp eq i32 [[PHI]], 456
 ; CHECK-NEXT:    ret i1 [[COMPARE]]
 ;
 entry:
@@ -46,10 +43,9 @@
 ; CHECK:       if.false:
 ; CHECK-NEXT:    br label [[MERGE]]
 ; CHECK:       merge:
-; CHECK-NEXT:    [[PHI:%.*]] = phi i32 [ 123, [[IF_TRUE]] ], [ 456, [[IF_FALSE]] ]
+; CHECK-NEXT:    [[COMPARE:%.*]] = phi i1 [ false, [[IF_FALSE]] ], [ true, [[IF_TRUE]] ]
 ; CHECK-NEXT:    br label [[EXIT:%.*]]
 ; CHECK:       exit:
-; CHECK-NEXT:    [[COMPARE:%.*]] = icmp ult i32 [[PHI]], 456
 ; CHECK-NEXT:    ret i1 [[COMPARE]]
 ;
 entry:
@@ -110,10 +106,9 @@
 ; CHECK:       if.false:
 ; CHECK-NEXT:    br label [[MERGE]]
 ; CHECK:       merge:
-; CHECK-NEXT:    [[PHI:%.*]] = phi i32 [ 123, [[IF_TRUE]] ], [ 456, [[IF_FALSE]] ]
+; CHECK-NEXT:    [[COMPARE:%.*]] = phi i1 [ false, [[IF_FALSE]] ], [ true, [[IF_TRUE]] ]
 ; CHECK-NEXT:    br label [[EXIT:%.*]]
 ; CHECK:       exit:
-; CHECK-NEXT:    [[COMPARE:%.*]] = icmp ne i32 [[PHI]], 456
 ; CHECK-NEXT:    ret i1 [[COMPARE]]
 ;
 entry:
@@ -133,3 +128,164 @@
   %compare = icmp ne i32 %phi, 456
   ret i1 %compare
 }
+
+define i1 @test_ne_undef(i1 %cond) {
+; CHECK-LABEL: @test_ne_undef(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    br i1 [[COND:%.*]], label [[IF_TRUE:%.*]], label [[IF_FALSE:%.*]]
+; CHECK:       if.true:
+; CHECK-NEXT:    br label [[MERGE:%.*]]
+; CHECK:       if.false:
+; CHECK-NEXT:    br label [[MERGE]]
+; CHECK:       merge:
+; CHECK-NEXT:    br label [[EXIT:%.*]]
+; CHECK:       exit:
+; CHECK-NEXT:    ret i1 false
+;
+entry:
+  br i1 %cond, label %if.true, label %if.false
+
+if.true:
+  br label %merge
+
+if.false:
+  br label %merge
+
+merge:
+  %phi = phi i32 [undef, %if.true], [456, %if.false]
+  br label %exit
+
+exit:
+  %compare = icmp ne i32 %phi, 456
+  ret i1 %compare
+}
+
+define <2 x i1> @test_ne_int_vector(i1 %cond) {
+; CHECK-LABEL: @test_ne_int_vector(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    br i1 [[COND:%.*]], label [[IF_TRUE:%.*]], label [[IF_FALSE:%.*]]
+; CHECK:       if.true:
+; CHECK-NEXT:    br label [[MERGE:%.*]]
+; CHECK:       if.false:
+; CHECK-NEXT:    br label [[MERGE]]
+; CHECK:       merge:
+; CHECK-NEXT:    [[COMPARE:%.*]] = phi <2 x i1> [ <i1 true, i1 false>, [[IF_FALSE]] ], [ <i1 false, i1 true>, [[IF_TRUE]] ]
+; CHECK-NEXT:    br label [[EXIT:%.*]]
+; CHECK:       exit:
+; CHECK-NEXT:    ret <2 x i1> [[COMPARE]]
+;
+entry:
+  br i1 %cond, label %if.true, label %if.false
+
+if.true:
+  br label %merge
+
+if.false:
+  br label %merge
+
+merge:
+  %phi = phi <2 x i32> [<i32 123, i32 123>, %if.true], [<i32 456, i32 456>, %if.false]
+  br label %exit
+
+exit:
+  %compare = icmp ne <2 x i32> %phi, <i32 123, i32 456>
+  ret <2 x i1> %compare
+}
+
+; TODO: We can also constant-fold this comparison for floats.
+define i1 @test_ne_float(i1 %cond) {
+; CHECK-LABEL: @test_ne_float(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    br i1 [[COND:%.*]], label [[IF_TRUE:%.*]], label [[IF_FALSE:%.*]]
+; CHECK:       if.true:
+; CHECK-NEXT:    br label [[MERGE:%.*]]
+; CHECK:       if.false:
+; CHECK-NEXT:    br label [[MERGE]]
+; CHECK:       merge:
+; CHECK-NEXT:    [[PHI:%.*]] = phi float [ 1.000000e+00, [[IF_TRUE]] ], [ 1.250000e+00, [[IF_FALSE]] ]
+; CHECK-NEXT:    br label [[EXIT:%.*]]
+; CHECK:       exit:
+; CHECK-NEXT:    [[COMPARE:%.*]] = fcmp one float [[PHI]], 1.250000e+00
+; CHECK-NEXT:    ret i1 [[COMPARE]]
+;
+entry:
+  br i1 %cond, label %if.true, label %if.false
+
+if.true:
+  br label %merge
+
+if.false:
+  br label %merge
+
+merge:
+  %phi = phi float [1.0, %if.true], [1.25, %if.false]
+  br label %exit
+
+exit:
+  %compare = fcmp one float %phi, 1.25
+  ret i1 %compare
+}
+
+define i1 @test_ne_float_undef(i1 %cond) {
+; CHECK-LABEL: @test_ne_float_undef(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    br i1 [[COND:%.*]], label [[IF_TRUE:%.*]], label [[IF_FALSE:%.*]]
+; CHECK:       if.true:
+; CHECK-NEXT:    br label [[MERGE:%.*]]
+; CHECK:       if.false:
+; CHECK-NEXT:    br label [[MERGE]]
+; CHECK:       merge:
+; CHECK-NEXT:    br label [[EXIT:%.*]]
+; CHECK:       exit:
+; CHECK-NEXT:    ret i1 true
+;
+entry:
+  br i1 %cond, label %if.true, label %if.false
+
+if.true:
+  br label %merge
+
+if.false:
+  br label %merge
+
+merge:
+  %phi = phi float [1.0, %if.true], [undef, %if.false]
+  br label %exit
+
+exit:
+  %compare = fcmp one float %phi, 1.25
+  ret i1 %compare
+}
+
+define <2 x i1> @test_ne_float_vector(i1 %cond) {
+; CHECK-LABEL: @test_ne_float_vector(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    br i1 [[COND:%.*]], label [[IF_TRUE:%.*]], label [[IF_FALSE:%.*]]
+; CHECK:       if.true:
+; CHECK-NEXT:    br label [[MERGE:%.*]]
+; CHECK:       if.false:
+; CHECK-NEXT:    br label [[MERGE]]
+; CHECK:       merge:
+; CHECK-NEXT:    [[PHI:%.*]] = phi <2 x float> [ <float 1.232500e+02, float 1.232500e+02>, [[IF_TRUE]] ], [ <float 4.562500e+02, float 4.562500e+02>, [[IF_FALSE]] ]
+; CHECK-NEXT:    br label [[EXIT:%.*]]
+; CHECK:       exit:
+; CHECK-NEXT:    [[COMPARE:%.*]] = fcmp one <2 x float> [[PHI]], <float 1.232500e+02, float 4.562500e+02>
+; CHECK-NEXT:    ret <2 x i1> [[COMPARE]]
+;
+entry:
+  br i1 %cond, label %if.true, label %if.false
+
+if.true:
+  br label %merge
+
+if.false:
+  br label %merge
+
+merge:
+  %phi = phi <2 x float> [<float 123.25, float 123.25>, %if.true], [<float 456.25, float 456.25>, %if.false]
+  br label %exit
+
+exit:
+  %compare = fcmp one <2 x float> %phi, <float 123.25, float 456.25>
+  ret <2 x i1> %compare
+}