[AMDGPU][MC] Corrected parsing of registers

Summary of changes:

refactored code for better readability and future improvements;
fixed bug 41281: https://bugs.llvm.org/show_bug.cgi?id=41281

Reviewers: artem.tamazov, arsenm

Differential Revision: https://reviews.llvm.org/D65224

llvm-svn: 373094