[AMDGPU][MC] Corrected parsing of FLAT offset modifier

Summary of changes:

- simplified handling of FLAT offset: offset_s13 and offset_u12 have been replaced with flat_offset;
- provided information about error position for pre-gfx9 targets;
- improved errors handling.

Reviewers: artem.tamazov, arsenm, rampitec

Differential Revision: https://reviews.llvm.org/D64244

llvm-svn: 365321