[llvm] [InstCombine] [X86] pblendvb intrinsics must be replaced by select when possible (PR #137322)

[Simon Pilgrim via llvm-commits]

================
@@ -801,6 +802,71 @@ bool VectorCombine::foldInsExtBinop(Instruction &I) {
   return true;
 }
 
+bool VectorCombine::foldBitOpOfBitcasts(Instruction &I) {
+  // Match: bitop(bitcast(x), bitcast(y)) -> bitcast(bitop(x, y))
+  auto *BinOp = dyn_cast<BinaryOperator>(&I);
+  if (!BinOp || !BinOp->isBitwiseLogicOp())
+    return false;
+
+  Value *LHS = BinOp->getOperand(0);
+  Value *RHS = BinOp->getOperand(1);
+
+  // Both operands must be bitcasts
+  auto *LHSCast = dyn_cast<BitCastInst>(LHS);
+  auto *RHSCast = dyn_cast<BitCastInst>(RHS);
+  if (!LHSCast || !RHSCast)
+    return false;
+
+  Value *LHSSrc = LHSCast->getOperand(0);
+  Value *RHSSrc = RHSCast->getOperand(0);
----------------
RKSimon wrote:

you should be able to update all the above to something like:
```
Value *LHSSrc, *RHSSrc;
if (!match(&I, m_BitwiseLogic(m_BitCast(m_Value(LHSSrc)),m_BitCast(m_Value(RHSSrc)))))
  return false;
```

https://github.com/llvm/llvm-project/pull/137322