[RISCV][NFC] Use defined variable instead some code.

Reviewed By: asb

Differential Revision: https://reviews.llvm.org/D130687

(cherry picked from commit e208bab)

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

@@ -8199,7 +8199,7 @@ static SDValue performSETCCCombine(SDNode *N, SelectionDAG &DAG,
   if (!isIntEqualitySetCC(Cond))
     return SDValue();
 
-  const APInt &C1 = cast<ConstantSDNode>(N1)->getAPIntValue();
+  const APInt &C1 = N1C->getAPIntValue();
 
   SDLoc dl(N);
   // If the constant is larger than 2^32 - 1 it is impossible for both sides