[RISCV] Split RISCVDisassembler::getInstruction into a 16-bit and 32-bit version. (#90254)

This reduces nesting of the common 32-bit case and makes it easier to
add longer instruction lengths in the future.

GitOrigin-RevId: 5569c219d35c78ad60aea127a06a51e202ae5b6f
1 file changed