[AMDGPU][llvm-mc] Fixes to support buffer atomics.

Fixes for MUBUF_Atomic instructions to make operand list valid:
 - For RTN insns, make a copy of $vdata_in operand as $vdata.
 - Do not add operand for GLC, it is hardcoded and comes as a token.
Workaround to avoid adding multiple default optional operands.
Tests added.

Differential Revision: http://reviews.llvm.org/D20257

llvm-svn: 270049