[RISCV] Use isNullConstant (NFC)

Author: kazutakahirata

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

@@ -12906,12 +12906,10 @@ static bool combine_CC(SDValue &LHS, SDValue &RHS, SDValue &CC, const SDLoc &DL,
   // shift can be omitted.
   // Fold setlt (sra X, N), 0 -> setlt X, 0 and
   // setge (sra X, N), 0 -> setge X, 0
-  if (auto *RHSConst = dyn_cast<ConstantSDNode>(RHS.getNode())) {
-    if ((CCVal == ISD::SETGE || CCVal == ISD::SETLT) &&
-        LHS.getOpcode() == ISD::SRA && RHSConst->isZero()) {
-      LHS = LHS.getOperand(0);
-      return true;
-    }
+  if (isNullConstant(RHS) && (CCVal == ISD::SETGE || CCVal == ISD::SETLT) &&
+      LHS.getOpcode() == ISD::SRA) {
+    LHS = LHS.getOperand(0);
+    return true;
   }
 
   if (!ISD::isIntEqualitySetCC(CCVal))