[AMDGPU][MC] Corrected MTBUF parsing and decoding

MTBUF implementation has many issues and this change addresses most of these:
- refactored duplicated code;
- hardcoded constants moved out of high-level code;
- fixed a decoding error when nfmt or dfmt are zero (bug 36932);
- corrected parsing of operand separators (bug 46403);
- corrected handling of missing operands (bug 46404);
- corrected handling of out-of-range modifiers (bug 46421);
- corrected default value (bug 46467).

Reviewers: arsenm, rampitec, vpykhtin, artem.tamazov, kzhuravl

Differential Revision: https://reviews.llvm.org/D83760