[InstCombineVectorOps] Use poison instead of undef as placeholder [NFC]

It's used to create a vector where only 1 element is used
While at it, change OOB extractelement to yield poison per LangRef

Author: nunoplopes

llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp

@@ -441,7 +441,7 @@ Instruction *InstCombinerImpl::visitExtractElementInst(ExtractElementInst &EI) {
         if (IndexC->getValue().getActiveBits() <= BitWidth)
           Idx = ConstantInt::get(Ty, IndexC->getValue().zextOrTrunc(BitWidth));
         else
-          Idx = UndefValue::get(Ty);
+          Idx = PoisonValue::get(Ty);
         return replaceInstUsesWith(EI, Idx);
       }
     }
@@ -2197,9 +2197,9 @@ static Instruction *canonicalizeInsertSplat(ShuffleVectorInst &Shuf,
       !match(Op1, m_Undef()) || match(Mask, m_ZeroMask()) || IndexC == 0)
     return nullptr;
 
-  // Insert into element 0 of an undef vector.
-  UndefValue *UndefVec = UndefValue::get(Shuf.getType());
-  Value *NewIns = Builder.CreateInsertElement(UndefVec, X, (uint64_t)0);
+  // Insert into element 0 of a poison vector.
+  PoisonValue *PoisonVec = PoisonValue::get(Shuf.getType());
+  Value *NewIns = Builder.CreateInsertElement(PoisonVec, X, (uint64_t)0);
 
   // Splat from element 0. Any mask element that is undefined remains undefined.
   // For example:

llvm/test/Transforms/InstCombine/insert-extract-shuffle-inseltpoison.ll

@@ -427,7 +427,7 @@ define <5 x float> @insert_not_undef_shuffle_translate_commute_lengthen(float %x
 
 define <4 x float> @insert_nonzero_index_splat(float %x) {
 ; CHECK-LABEL: @insert_nonzero_index_splat(
-; CHECK-NEXT:    [[TMP1:%.*]] = insertelement <4 x float> undef, float [[X:%.*]], i64 0
+; CHECK-NEXT:    [[TMP1:%.*]] = insertelement <4 x float> poison, float [[X:%.*]], i64 0
 ; CHECK-NEXT:    [[SPLAT:%.*]] = shufflevector <4 x float> [[TMP1]], <4 x float> poison, <4 x i32> <i32 poison, i32 0, i32 0, i32 poison>
 ; CHECK-NEXT:    ret <4 x float> [[SPLAT]]
 ;
@@ -438,7 +438,7 @@ define <4 x float> @insert_nonzero_index_splat(float %x) {
 
 define <3 x double> @insert_nonzero_index_splat_narrow(double %x) {
 ; CHECK-LABEL: @insert_nonzero_index_splat_narrow(
-; CHECK-NEXT:    [[TMP1:%.*]] = insertelement <3 x double> undef, double [[X:%.*]], i64 0
+; CHECK-NEXT:    [[TMP1:%.*]] = insertelement <3 x double> poison, double [[X:%.*]], i64 0
 ; CHECK-NEXT:    [[SPLAT:%.*]] = shufflevector <3 x double> [[TMP1]], <3 x double> poison, <3 x i32> <i32 0, i32 poison, i32 0>
 ; CHECK-NEXT:    ret <3 x double> [[SPLAT]]
 ;
@@ -449,7 +449,7 @@ define <3 x double> @insert_nonzero_index_splat_narrow(double %x) {
 
 define <5 x i7> @insert_nonzero_index_splat_widen(i7 %x) {
 ; CHECK-LABEL: @insert_nonzero_index_splat_widen(
-; CHECK-NEXT:    [[TMP1:%.*]] = insertelement <5 x i7> undef, i7 [[X:%.*]], i64 0
+; CHECK-NEXT:    [[TMP1:%.*]] = insertelement <5 x i7> poison, i7 [[X:%.*]], i64 0
 ; CHECK-NEXT:    [[SPLAT:%.*]] = shufflevector <5 x i7> [[TMP1]], <5 x i7> poison, <5 x i32> <i32 poison, i32 0, i32 0, i32 poison, i32 0>
 ; CHECK-NEXT:    ret <5 x i7> [[SPLAT]]
 ;

llvm/test/Transforms/InstCombine/insert-extract-shuffle.ll

@@ -427,7 +427,7 @@ define <5 x float> @insert_not_undef_shuffle_translate_commute_lengthen(float %x
 
 define <4 x float> @insert_nonzero_index_splat(float %x) {
 ; CHECK-LABEL: @insert_nonzero_index_splat(
-; CHECK-NEXT:    [[TMP1:%.*]] = insertelement <4 x float> undef, float [[X:%.*]], i64 0
+; CHECK-NEXT:    [[TMP1:%.*]] = insertelement <4 x float> poison, float [[X:%.*]], i64 0
 ; CHECK-NEXT:    [[SPLAT:%.*]] = shufflevector <4 x float> [[TMP1]], <4 x float> poison, <4 x i32> <i32 poison, i32 0, i32 0, i32 poison>
 ; CHECK-NEXT:    ret <4 x float> [[SPLAT]]
 ;
@@ -438,7 +438,7 @@ define <4 x float> @insert_nonzero_index_splat(float %x) {
 
 define <3 x double> @insert_nonzero_index_splat_narrow(double %x) {
 ; CHECK-LABEL: @insert_nonzero_index_splat_narrow(
-; CHECK-NEXT:    [[TMP1:%.*]] = insertelement <3 x double> undef, double [[X:%.*]], i64 0
+; CHECK-NEXT:    [[TMP1:%.*]] = insertelement <3 x double> poison, double [[X:%.*]], i64 0
 ; CHECK-NEXT:    [[SPLAT:%.*]] = shufflevector <3 x double> [[TMP1]], <3 x double> poison, <3 x i32> <i32 0, i32 poison, i32 0>
 ; CHECK-NEXT:    ret <3 x double> [[SPLAT]]
 ;
@@ -449,7 +449,7 @@ define <3 x double> @insert_nonzero_index_splat_narrow(double %x) {
 
 define <5 x i7> @insert_nonzero_index_splat_widen(i7 %x) {
 ; CHECK-LABEL: @insert_nonzero_index_splat_widen(
-; CHECK-NEXT:    [[TMP1:%.*]] = insertelement <5 x i7> undef, i7 [[X:%.*]], i64 0
+; CHECK-NEXT:    [[TMP1:%.*]] = insertelement <5 x i7> poison, i7 [[X:%.*]], i64 0
 ; CHECK-NEXT:    [[SPLAT:%.*]] = shufflevector <5 x i7> [[TMP1]], <5 x i7> poison, <5 x i32> <i32 poison, i32 0, i32 0, i32 poison, i32 0>
 ; CHECK-NEXT:    ret <5 x i7> [[SPLAT]]
 ;

llvm/test/Transforms/InstCombine/vscale_extractelement.ll

@@ -217,12 +217,12 @@ entry:
   ret i64 %1
 }
 
-; Check that undef is returned when the extracted element has wrapped.
+; Check that poison is returned when the extracted element has wrapped.
 
 define i8 @ext_lane256_from_stepvec() {
 ; CHECK-LABEL: @ext_lane256_from_stepvec(
 ; CHECK-NEXT:  entry:
-; CHECK-NEXT:    ret i8 undef
+; CHECK-NEXT:    ret i8 poison
 ;
 entry:
   %0 = call <vscale x 512 x i8> @llvm.experimental.stepvector.nxv512i8()