[RISCV] Remove MCRegisterInfo dependency from compressInst/uncompresInst/isCompressib...

[RISCV] Remove MCRegisterInfo dependency from compressInst/uncompresInst/isCompressibleInst.

This was being used to lookup the register class for a register number,
but those live in a tablegened array. We can index that array directly
just like RISCVAsmParser does.

Differential Revision: https://reviews.llvm.org/D141951