[InstCombine] Require poison operand in canEvaluateShuffled transform

This transform works on single-source shuffles, which require that
the second operand is poison, not undef. Otherwise we may convert
undef to poison.

Fixes #92887.

Author: nikic

llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp

@@ -2901,7 +2901,7 @@ Instruction *InstCombinerImpl::visitShuffleVectorInst(ShuffleVectorInst &SVI) {
   if (Instruction *I = foldIdentityPaddedShuffles(SVI))
     return I;
 
-  if (match(RHS, m_Undef()) && canEvaluateShuffled(LHS, Mask)) {
+  if (match(RHS, m_Poison()) && canEvaluateShuffled(LHS, Mask)) {
     Value *V = evaluateInDifferentElementOrder(LHS, Mask, Builder);
     return replaceInstUsesWith(SVI, V);
   }

llvm/test/Transforms/InstCombine/vec_shuffle.ll

@@ -2331,10 +2331,9 @@ define <4 x i16> @blend_elements_from_load(ptr align 8 %_0) {
   ret <4 x i16> %rv
 }
 
-; FIXME: This is a miscompile.
 define i16 @pr92887(<2 x i16> %v) {
 ; CHECK-LABEL: @pr92887(
-; CHECK-NEXT:    ret i16 poison
+; CHECK-NEXT:    ret i16 undef
 ;
   %v0 = extractelement <2 x i16> %v, i64 0
   %v0lo = and i16 %v0, 1