[RISCV][NFC] Make logic in RISCVMCCodeEmitter::getImmOpValue more defensive
As pointed out by @sabuasal in a comment on D23568, the logic in
RISCVMCCodeEmitter::getImmOpValue could be more defensive. Although with the
current instruction definitions it is always the case that VK_RISCV_LO is
always used with either an I- or S-format instruction, this may not always be
the case in the future. Add a check to ensure we will get an assertion in
debug builds if that changes.