[AMDGPU][MC] Refactored exp tgt handling

Summary:
- Separated tgt encoding from parsing;
- Separated tgt decoding from printing;
- Improved errors handling;
- Disabled leading zeroes in index. The following code is no longer accepted: exp pos00 v3, v2, v1, v0

Reviewers: arsenm, rampitec, foad

Differential Revision: https://reviews.llvm.org/D95216