[AArch64][SVE] Change placeholder from undef to poison

Default to a `poison` vector when calling `@llvm.vector.insert`.

Author: pedroclobo

llvm/lib/Target/AArch64/AArch64ISelLowering.cpp

@@ -17504,7 +17504,7 @@ bool AArch64TargetLowering::lowerInterleavedStore(StoreInst *SI,
 
       if (UseScalable)
         Shuffle = Builder.CreateInsertVector(
-            STVTy, UndefValue::get(STVTy), Shuffle,
+            STVTy, PoisonValue::get(STVTy), Shuffle,
             ConstantInt::get(Type::getInt64Ty(STVTy->getContext()), 0));
 
       Ops.push_back(Shuffle);