[InstCombine] Remove dead poison check. NFCI (#141264)

As far as I understand any binary op with poison as either operand will
constant fold to poison, so this check will never trigger.
`llvm::ConstantFoldBinaryInstruction` seems to confirm this?

I think this ended up getting left behind because originally
shufflevectors with undef indices produced undef elements, and we
couldn't pull the shuffle across some binops like `or undef, -1 --> -1`.

This code was added in 8c65515 to
partially fix it and further extended in
f749901, originally checking for undef
but changed to check for poison in cd54c47

But nowadays shufflevectors with undef indices are treated as poison
indices as of 575fdea, and so produce
poison elements, so this is no longer an issue

Author: lukel97

llvm/lib/Transforms/InstCombine/InstructionCombining.cpp

@@ -2254,25 +2254,6 @@ Instruction *InstCombinerImpl::foldVectorBinop(BinaryOperator &Inst) {
         }
         NewVecC[ShMask[I]] = CElt;
       }
-      // If this is a widening shuffle, we must be able to extend with poison
-      // elements. If the original binop does not produce a poison in the high
-      // lanes, then this transform is not safe.
-      // Similarly for poison lanes due to the shuffle mask, we can only
-      // transform binops that preserve poison.
-      // TODO: We could shuffle those non-poison constant values into the
-      //       result by using a constant vector (rather than an poison vector)
-      //       as operand 1 of the new binop, but that might be too aggressive
-      //       for target-independent shuffle creation.
-      if (I >= SrcVecNumElts || ShMask[I] < 0) {
-        Constant *MaybePoison =
-            ConstOp1
-                ? ConstantFoldBinaryOpOperands(Opcode, PoisonScalar, CElt, DL)
-                : ConstantFoldBinaryOpOperands(Opcode, CElt, PoisonScalar, DL);
-        if (!MaybePoison || !isa<PoisonValue>(MaybePoison)) {
-          MayChange = false;
-          break;
-        }
-      }
     }
     if (MayChange) {
       Constant *NewC = ConstantVector::get(NewVecC);

llvm/test/Transforms/InstCombine/vec_shuffle.ll

@@ -732,8 +732,11 @@ define <4 x i16> @widening_shuffle_shl_constant_op1_non0(<2 x i16> %v) {
   ret <4 x i16> %bo
 }
 
-; A binop that does not produce undef in the high lanes can not be moved before the shuffle.
-; This is not ok because 'or -1, undef --> -1' but moving the shuffle results in undef instead.
+; Previously, a shufflevector would produce an undef element from an undef mask
+; index, which meant that pulling the shuffle out wasn't correct if the original
+; binary op produced a non-undef result, e.g. or -1, undef --> -1.
+;
+; However nowadays shufflevector produces poison, which is safe to propagate.
 
 define <4 x i16> @widening_shuffle_or(<2 x i16> %v) {
 ; CHECK-LABEL: @widening_shuffle_or(