[AMDGPU][MC][NFC] MUBUF/MTBUF code cleanup

Refactor code to reduce code duplication and improve maintainability.

- Extract BUF_Pseudo common base class
- Refactor getMUBUFInsDA
- Refactor getMUBUFAtomicInsDA
- Refactor getMTBUFInsDA
- Refactor getMUBUFAsmOps
- Refactor getMTBUFAsmOps

Differential Revision: https://reviews.llvm.org/D140410