[llvm] Optimize count leading ones if promoted type (PR #99591)

[Simon Pilgrim via llvm-commits]

================
@@ -5049,6 +5049,40 @@ static MVT getPromotedVectorElementType(const TargetLowering &TLI,
   return MidVT;
 }
 
+// (CTLZ (XOR Op -1)) --> (TRUNCATE (CTLZ_ZERO_UNDEF
+//                                    (XOR (SHIFT (ANYEXTEND Op1)
+//                                                ShiftAmount)
+//                                         -1)))
+static bool ExtendCtlzNot(SDNode *Node, SDValue &Result, SDLoc &dl, MVT OVT,
+                          MVT NVT, SelectionDAG &DAG) {
+  SDValue NotOp = Node->getOperand(0);
+  if (NotOp.getOpcode() != ISD::XOR)
+    return false;
+
+  SDValue SrcOp = NotOp->getOperand(0);
+  SDValue CstOp = NotOp->getOperand(1);
+
+  ConstantSDNode *Cst = dyn_cast<ConstantSDNode>(CstOp);
+
+  if (!Cst || !Cst->isAllOnes())
+    return false;
----------------
RKSimon wrote:

We can now use SDPatternMatch to do this for us - 
```
sd_match(Node->getOperand(0), m_Not(SrcOp))
```

https://github.com/llvm/llvm-project/pull/99591