[llvm] [PowerPC] Combine sub within setcc back to sext (PR #66978)

[Amy Kwan via llvm-commits]

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@@ -14425,15 +14428,52 @@ SDValue PPCTargetLowering::combineSetCC(SDNode *N,
     // x != 0-y --> x+y != 0
     if (RHS.getOpcode() == ISD::SUB && isNullConstant(RHS.getOperand(0)) &&
         RHS.hasOneUse()) {
-      SDLoc DL(N);
-      SelectionDAG &DAG = DCI.DAG;
-      EVT VT = N->getValueType(0);
-      EVT OpVT = LHS.getValueType();
       SDValue Add = DAG.getNode(ISD::ADD, DL, OpVT, LHS, RHS.getOperand(1));
       return DAG.getSetCC(DL, VT, Add, DAG.getConstant(0, DL, OpVT), CC);
     }
   }
 
+  if (CC == ISD::SETULT && isa<ConstantSDNode>(RHS)) {
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amy-kwan wrote:

nit: I think it is not ideal to have `isa<>` followed by `cast<>`?

https://github.com/llvm/llvm-project/pull/66978