[Hexagon] Change placeholder from undef to poison (#135555)

A `poison` vector should be used when performing an identity shuffle.

Co-authored-by: Nuno Lopes <[email protected]>

Author: pedroclobo

llvm/lib/Target/Hexagon/HexagonVectorCombine.cpp

@@ -2318,9 +2318,9 @@ auto HexagonVectorCombine::insertb(IRBuilderBase &Builder, Value *Dst,
   assert(0 <= Where && Where + Length <= DstLen);
 
   int P2Len = PowerOf2Ceil(SrcLen | DstLen);
-  auto *Undef = UndefValue::get(getByteTy());
-  Value *P2Src = vresize(Builder, Src, P2Len, Undef);
-  Value *P2Dst = vresize(Builder, Dst, P2Len, Undef);
+  auto *Poison = PoisonValue::get(getByteTy());
+  Value *P2Src = vresize(Builder, Src, P2Len, Poison);
+  Value *P2Dst = vresize(Builder, Dst, P2Len, Poison);
 
   SmallVector<int, 256> SMask(P2Len);
   for (int i = 0; i != P2Len; ++i) {
@@ -2331,7 +2331,7 @@ auto HexagonVectorCombine::insertb(IRBuilderBase &Builder, Value *Dst,
   }
 
   Value *P2Insert = Builder.CreateShuffleVector(P2Dst, P2Src, SMask, "shf");
-  return vresize(Builder, P2Insert, DstLen, Undef);
+  return vresize(Builder, P2Insert, DstLen, Poison);
 }
 
 auto HexagonVectorCombine::vlalignb(IRBuilderBase &Builder, Value *Lo,