Improve undef/poison usage in test (per PR automation failure), and using meaningful global instead of magic constant for shufflemask poison-indicator

Author: agorenstein-nvidia

llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp

@@ -545,7 +545,7 @@ Instruction *InstCombinerImpl::visitExtractElementInst(ExtractElementInst &EI) {
       // extractelt (shufflevector %v1, %v2, splat-mask) idx ->
       // extractelt %v1, splat-mask[0]
       auto mask = SVI->getShuffleMask();
-      if (mask[0] != -1 && all_equal(mask))
+      if (mask[0] != PoisonMaskElem && all_equal(mask))
         return ExtractElementInst::Create(SVI->getOperand(0),
                                           Builder.getInt64(mask[0]));
 

llvm/test/Transforms/InstCombine/vec_extract_through_broadcast.ll

@@ -37,7 +37,7 @@ define float @no_extract_from_general_no_splat_0(<2 x float> %1, i64 %idx) {
 ; CHECK-NEXT:    [[TMP2:%.*]] = extractelement <4 x float> [[TMP1]], i64 %idx
 ; CHECK-NEXT:    ret float [[TMP2]]
 ;
-  %3 = shufflevector <2 x float> %1, <2 x float> poison, <4 x i32> <i32 undef, i32 1, i32 1, i32 1>
+  %3 = shufflevector <2 x float> %1, <2 x float> poison, <4 x i32> <i32 poison, i32 1, i32 1, i32 1>
   %4 = extractelement <4 x float> %3, i64 %idx
   ret float %4
 }
@@ -48,7 +48,7 @@ define float @no_extract_from_general_no_splat_1(<2 x float> %1, i64 %idx) {
 ; CHECK-NEXT:    [[TMP2:%.*]] = extractelement <4 x float> [[TMP1]], i64 %idx
 ; CHECK-NEXT:    ret float [[TMP2]]
 ;
-  %3 = shufflevector <2 x float> %1, <2 x float> poison, <4 x i32> <i32 1, i32 undef, i32 1, i32 1>
+  %3 = shufflevector <2 x float> %1, <2 x float> poison, <4 x i32> <i32 1, i32 poison, i32 1, i32 1>
   %4 = extractelement <4 x float> %3, i64 %idx
   ret float %4
 }