[X86] combineX86AddSub - pull out repeated getOperand() call. NFC.

Author: RKSimon

llvm/lib/Target/X86/X86ISelLowering.cpp

@@ -57447,7 +57447,7 @@ static SDValue combineX86AddSub(SDNode *N, SelectionDAG &DAG,
   bool IsSub = X86ISD::SUB == N->getOpcode();
   unsigned GenericOpc = IsSub ? ISD::SUB : ISD::ADD;
 
-  if (IsSub && isOneConstant(N->getOperand(1)) && !N->hasAnyUseOfValue(0))
+  if (IsSub && isOneConstant(RHS) && !N->hasAnyUseOfValue(0))
     if (SDValue CMP = combineX86SubCmpForFlags(N, SDValue(N, 1), DAG, DCI, ST))
       return CMP;