[InstCombine] Match poison instead of undef in binop of same-mask shuffle fold

Author: nikic

llvm/lib/Transforms/InstCombine/InstructionCombining.cpp

@@ -1728,8 +1728,8 @@ Instruction *InstCombinerImpl::foldVectorBinop(BinaryOperator &Inst) {
 
   // If both arguments of the binary operation are shuffles that use the same
   // mask and shuffle within a single vector, move the shuffle after the binop.
-  if (match(LHS, m_Shuffle(m_Value(V1), m_Undef(), m_Mask(Mask))) &&
-      match(RHS, m_Shuffle(m_Value(V2), m_Undef(), m_SpecificMask(Mask))) &&
+  if (match(LHS, m_Shuffle(m_Value(V1), m_Poison(), m_Mask(Mask))) &&
+      match(RHS, m_Shuffle(m_Value(V2), m_Poison(), m_SpecificMask(Mask))) &&
       V1->getType() == V2->getType() &&
       (LHS->hasOneUse() || RHS->hasOneUse() || LHS == RHS)) {
     // Op(shuffle(V1, Mask), shuffle(V2, Mask)) -> shuffle(Op(V1, V2), Mask)