[RISCV] Fix -Wunused-variable in RISCVISelLowering.cpp (NFC)

llvm-project/llvm/lib/Target/RISCV/RISCVISelLowering.cpp:13754:12:
error: unused variable 'Opc' [-Werror,-Wunused-variable]
  unsigned Opc = N->getOpcode();
           ^
1 error generated.