[PatternMatch] Do not accept undef elements in m_AllOnes() and friends (#88217)

Change all the cstval_pred_ty based PatternMatch helpers (things like
m_AllOnes and m_Zero) to only allow poison elements inside vector
splats, not undef elements.

Historically, we used to represent non-demanded elements in vectors
using undef. Nowadays, we use poison instead. As such, I believe that
support for undef in vector splats is no longer useful.

At the same time, while poison splat elements are pretty much always
safe to ignore, this is not generally the case for undef elements. We
have existing miscompiles in our tests due to this (see the
masked-merge-*.ll tests changed here) and it's easy to miss such cases
in the future, now that we write tests using poison instead of undef
elements.

I think overall, keeping support for undef elements no longer makes
sense, and we should drop it. Once this is done consistently, I think we
may also consider allowing poison in m_APInt by default, as doing that
change is much less risky than doing the same with undef.

This change involves a substantial amount of test changes. For most
tests, I've just replaced undef with poison, as I don't think there is
value in retaining both. For some tests (where the distinction between
undef and poison is important), I've duplicated tests.

1 files changed, 1 insertions, 1 deletions

diff --git a/llvm/lib/IR/Constants.cpp b/llvm/lib/IR/Constants.cpp
index a5fb497..45b359a 100644
--- a/llvm/lib/IR/Constants.cpp
+++ b/llvm/lib/IR/Constants.cpp
@@ -316,7 +316,7 @@ bool Constant::isElementWiseEqual(Value *Y) const {
   Constant *C0 = ConstantExpr::getBitCast(const_cast<Constant *>(this), IntTy);
   Constant *C1 = ConstantExpr::getBitCast(cast<Constant>(Y), IntTy);
   Constant *CmpEq = ConstantExpr::getICmp(ICmpInst::ICMP_EQ, C0, C1);
-  return isa<UndefValue>(CmpEq) || match(CmpEq, m_One());
+  return isa<PoisonValue>(CmpEq) || match(CmpEq, m_One());
 }
 
 static bool