[AMDGPU][llvm-mc] Support for 32-bit inline literals

Patch by: Artem Tamazov

Summary:
Note: Support for 64-bit inline literals TBD
Added: Support of abs/neg modifiers for literals (incomplete; parsing TBD).
Added: Some TODO comments.
Reworked/clarity: rename isInlineImm() to isInlinableImm()
Reworked/robustness: disallow BitsToFloat() with undefined value in isInlinableImm()
Reworked/reuse: isSSrc32/64(), isVSrc32/64()
Tests added.

Reviewers: tstellarAMD, arsenm

Subscribers: vpykhtin, nhaustov, SamWot, arsenm

Projects: #llvm-amdgpu-spb

Differential Revision: http://reviews.llvm.org/D17204

llvm-svn: 261559