[RISCV] Use getConstantOperandVal. NFC

Author: topperc

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

@@ -5009,8 +5009,7 @@ SDValue RISCVTargetLowering::LowerIS_FPCLASS(SDValue Op,
   SDLoc DL(Op);
   MVT VT = Op.getSimpleValueType();
   MVT XLenVT = Subtarget.getXLenVT();
-  auto CNode = cast<ConstantSDNode>(Op.getOperand(1));
-  unsigned Check = CNode->getZExtValue();
+  unsigned Check = Op.getConstantOperandVal(1);
   unsigned TDCMask = 0;
   if (Check & fcSNan)
     TDCMask |= RISCV::FPMASK_Signaling_NaN;