[AMDGPU][llvm-mc] Quickfix for r272748 to enable labels in branch instructions.

Fixes issue mentioned at:
  https://github.com/RadeonOpenCompute/LLVM-AMDGPU-Assembler-Extra/issues/13.
Lit tests added.

Differential Revision: http://reviews.llvm.org/D22133

llvm-svn: 275054