[AMDGPU][MC] Corrected parser to avoid generation of excessive error messages

Summary of changes:
- Changed parser to eliminate generation of excessive error messages;
- Corrected lit tests to match all expected error messages;
- Corrected lit tests to guard against unwanted extra messages (added option "--implicit-check-not=error:");
- Added missing checks and fixed some typos in tests.

See bug 46907: https://bugs.llvm.org/show_bug.cgi?id=46907

Reviewers: arsenm, rampitec

Differential Revision: https://reviews.llvm.org/D86940