[AMDGPU][NFCI] Refactor BUFInstructions.td (#65746)

Make the code more consistent:
- More pattern classes follow the simpler interface with string instead
of pseudos.
- CmpSwap patterns are encapsulated in SIBufferAtomicCmpSwapPat.
- Pseudo store patterns are separated out, similarly to the load
counterparts.
- MUBUF_Offset_Load_Pat is now GCNPat, as others.