[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.

llvm-svn: 325775