[RISCV] Rework how implied SP operands work in the disassembler. NFC

Previously we added the SP operands when an immediate operand was added
to certain opcodes.

This patch moves it to a post processing step using the information
in MCInstrDesc. This avoids an explicit opcode list in RISCVDisassembler.cpp.

In considered using a custom DecoderMethod, but the bit swizzling we
need to do for the immediates on these instructions made that
unattractive.

Reviewed By: asb

Differential Revision: https://reviews.llvm.org/D149931