[InstCombine] Check for poison instead of undef in shuffle of unop transform

Otherwise this may not actually be a single-source shuffle.

Author: nikic

llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp

@@ -2462,7 +2462,7 @@ static Instruction *foldShuffleOfUnaryOps(ShuffleVectorInst &Shuf,
 
   // Match 1-input (unary) shuffle.
   // shuffle (fneg/fabs X), Mask --> fneg/fabs (shuffle X, Mask)
-  if (S0->hasOneUse() && match(Shuf.getOperand(1), m_Undef())) {
+  if (S0->hasOneUse() && match(Shuf.getOperand(1), m_Poison())) {
     Value *NewShuf = Builder.CreateShuffleVector(X, Shuf.getShuffleMask());
     if (IsFNeg)
       return UnaryOperator::CreateFNegFMF(NewShuf, S0);

llvm/test/Transforms/InstCombine/vec_shuffle.ll

@@ -1280,12 +1280,11 @@ define <2 x float> @fneg(<2 x float> %x) {
   ret <2 x float> %r
 }
 
-; FIXME: This is a miscompile.
 define <2 x float> @fneg_not_single_source(<2 x float> %x) {
 ; CHECK-LABEL: @fneg_not_single_source(
-; CHECK-NEXT:    [[TMP1:%.*]] = shufflevector <2 x float> [[X:%.*]], <2 x float> poison, <2 x i32> <i32 0, i32 poison>
-; CHECK-NEXT:    [[SPLAT:%.*]] = fneg <2 x float> [[TMP1]]
-; CHECK-NEXT:    ret <2 x float> [[SPLAT]]
+; CHECK-NEXT:    [[NEG:%.*]] = fneg <2 x float> [[X:%.*]]
+; CHECK-NEXT:    [[SPLAT1:%.*]] = insertelement <2 x float> [[NEG]], float undef, i64 1
+; CHECK-NEXT:    ret <2 x float> [[SPLAT1]]
 ;
   %neg = fneg <2 x float> %x
   %splat = shufflevector <2 x float> %neg, <2 x float> undef, <2 x i32> <i32 0, i32 2>