[AMDGPU][llvm-mc] Support of Trap Handler registers (TTMP0..11 and TBA/TMA)git status

Tests added along with implemented feature.
Note that there is a small leftover of unecessary MI sheduling issue
(more info in the review). CodeGen/AMDGPU/salu-to-valu.ll updated to fix
the false regression.

TODO: Support for TTMP quads, comma-separated syntax in "[]" and more.

Differential Revision: http://reviews.llvm.org/D17825

llvm-svn: 266205