[RISCV] Refactor RISCVDisassembler::getInstruction to remove repeated code. NFC

For 4 byte instructions we were always setting size to 4 eventually. Same
for 2 byte instructions. So do it as soon as we know the from the opcode.

Add a return to the end of the 4 byte code so we don't have to have an else
around the 2 byte code.

Differential Revision: https://reviews.llvm.org/D143445