[VectorCombine] Use poison insteaf of undef as placeholder [NFC]

These vector lanes are never accessed. They are used for shifting a value into the right lane
and therefore only 1 value of the whole vector is actually used

Author: nunoplopes

llvm/lib/Transforms/Vectorize/VectorCombine.cpp

@@ -457,9 +457,9 @@ bool VectorCombine::isExtractExtractCheap(ExtractElementInst *Ext0,
 
     // If we are extracting from 2 different indexes, then one operand must be
     // shuffled before performing the vector operation. The shuffle mask is
-    // undefined except for 1 lane that is being translated to the remaining
+    // poison except for 1 lane that is being translated to the remaining
     // extraction lane. Therefore, it is a splat shuffle. Ex:
-    // ShufMask = { undef, undef, 0, undef }
+    // ShufMask = { poison, poison, 0, poison }
     // TODO: The cost model has an option for a "broadcast" shuffle
     //       (splat-from-element-0), but no option for a more general splat.
     NewCost +=
@@ -476,9 +476,9 @@ bool VectorCombine::isExtractExtractCheap(ExtractElementInst *Ext0,
 /// to a new element location.
 static Value *createShiftShuffle(Value *Vec, unsigned OldIndex,
                                  unsigned NewIndex, IRBuilder<> &Builder) {
-  // The shuffle mask is undefined except for 1 lane that is being translated
+  // The shuffle mask is poison except for 1 lane that is being translated
   // to the new element index. Example for OldIndex == 2 and NewIndex == 0:
-  // ShufMask = { 2, undef, undef, undef }
+  // ShufMask = { 2, poison, poison, poison }
   auto *VecTy = cast<FixedVectorType>(Vec->getType());
   SmallVector<int, 32> ShufMask(VecTy->getNumElements(), PoisonMaskElem);
   ShufMask[NewIndex] = OldIndex;
@@ -929,7 +929,7 @@ bool VectorCombine::foldExtractedCmps(Instruction &I) {
 
   // Create a vector constant from the 2 scalar constants.
   SmallVector<Constant *, 32> CmpC(VecTy->getNumElements(),
-                                   UndefValue::get(VecTy->getElementType()));
+                                   PoisonValue::get(VecTy->getElementType()));
   CmpC[Index0] = C0;
   CmpC[Index1] = C1;
   Value *VCmp = Builder.CreateCmp(Pred, X, ConstantVector::get(CmpC));

llvm/test/Transforms/VectorCombine/X86/extract-cmp-binop.ll

@@ -36,7 +36,7 @@ define i1 @fcmp_or_v4f64(<4 x double> %a) {
 ; SSE-NEXT:    ret i1 [[R]]
 ;
 ; AVX-LABEL: @fcmp_or_v4f64(
-; AVX-NEXT:    [[TMP1:%.*]] = fcmp olt <4 x double> [[A:%.*]], <double 4.200000e+01, double undef, double -8.000000e+00, double undef>
+; AVX-NEXT:    [[TMP1:%.*]] = fcmp olt <4 x double> [[A:%.*]], <double 4.200000e+01, double poison, double -8.000000e+00, double poison>
 ; AVX-NEXT:    [[SHIFT:%.*]] = shufflevector <4 x i1> [[TMP1]], <4 x i1> poison, <4 x i32> <i32 2, i32 poison, i32 poison, i32 poison>
 ; AVX-NEXT:    [[TMP2:%.*]] = or <4 x i1> [[TMP1]], [[SHIFT]]
 ; AVX-NEXT:    [[R:%.*]] = extractelement <4 x i1> [[TMP2]], i64 0
@@ -52,7 +52,7 @@ define i1 @fcmp_or_v4f64(<4 x double> %a) {
 
 define i1 @icmp_xor_v4i32(<4 x i32> %a) {
 ; CHECK-LABEL: @icmp_xor_v4i32(
-; CHECK-NEXT:    [[TMP1:%.*]] = icmp sgt <4 x i32> [[A:%.*]], <i32 undef, i32 -8, i32 undef, i32 42>
+; CHECK-NEXT:    [[TMP1:%.*]] = icmp sgt <4 x i32> [[A:%.*]], <i32 poison, i32 -8, i32 poison, i32 42>
 ; CHECK-NEXT:    [[SHIFT:%.*]] = shufflevector <4 x i1> [[TMP1]], <4 x i1> poison, <4 x i32> <i32 poison, i32 3, i32 poison, i32 poison>
 ; CHECK-NEXT:    [[TMP2:%.*]] = xor <4 x i1> [[TMP1]], [[SHIFT]]
 ; CHECK-NEXT:    [[R:%.*]] = extractelement <4 x i1> [[TMP2]], i64 1
@@ -78,7 +78,7 @@ define i1 @icmp_add_v8i32(<8 x i32> %a) {
 ; SSE-NEXT:    ret i1 [[R]]
 ;
 ; AVX-LABEL: @icmp_add_v8i32(
-; AVX-NEXT:    [[TMP1:%.*]] = icmp eq <8 x i32> [[A:%.*]], <i32 undef, i32 undef, i32 -8, i32 undef, i32 undef, i32 undef, i32 undef, i32 42>
+; AVX-NEXT:    [[TMP1:%.*]] = icmp eq <8 x i32> [[A:%.*]], <i32 poison, i32 poison, i32 -8, i32 poison, i32 poison, i32 poison, i32 poison, i32 42>
 ; AVX-NEXT:    [[SHIFT:%.*]] = shufflevector <8 x i1> [[TMP1]], <8 x i1> poison, <8 x i32> <i32 poison, i32 poison, i32 7, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison>
 ; AVX-NEXT:    [[TMP2:%.*]] = add <8 x i1> [[TMP1]], [[SHIFT]]
 ; AVX-NEXT:    [[R:%.*]] = extractelement <8 x i1> [[TMP2]], i64 2