[RISCV] Use decodeUImmLog2XLenNonZeroOperand in decodeRVCInstrRdRs1UImm. NFC (#133759)
decodeUImmLog2XLenNonZeroOperand already contains the uimm5 check for
RV32 so we can reuse it. This makes C_SLLI_HINT code more similar to the
tblgen code for C_SLLI.