[InstCombine] Check for poison instead of undef in shuffle transform

This one doesn't seem to make a practical difference because we'd
canonicalize undef -> poison in the relevant cases anywy.

Author: nikic

llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp

@@ -777,10 +777,10 @@ static ShuffleOps collectShuffleElements(Value *V, SmallVectorImpl<int> &Mask,
   assert(V->getType()->isVectorTy() && "Invalid shuffle!");
   unsigned NumElts = cast<FixedVectorType>(V->getType())->getNumElements();
 
-  if (match(V, m_Undef())) {
+  if (match(V, m_Poison())) {
     Mask.assign(NumElts, -1);
     return std::make_pair(
-        PermittedRHS ? UndefValue::get(PermittedRHS->getType()) : V, nullptr);
+        PermittedRHS ? PoisonValue::get(PermittedRHS->getType()) : V, nullptr);
   }
 
   if (isa<ConstantAggregateZero>(V)) {