AMDGPU/SI: Assembler: improvements to support trap handlers.

Add ParseAMDGPURegister which can be invoked recursively for parsing lists.
Rename getRegForName to getSpecialRegForName.
Support legacy SP3 register list syntax: [s2,s3,s4,s5] or [flat_scratch_lo,flat_scratch_hi].
Add 64-bit registers TBA, TMA where missing.
Add some tests.

Differential Revision: http://reviews.llvm.org/D19163

llvm-svn: 266865