AMDGPU/AsmParser: Add support for parsing symbol operands

Summary:
We can now reference symbols directly in operands, like this:
s_mov_b32 s0, global

Reviewers: artem.tamazov, vpykhtin, SamWot, nhaustov

Subscribers: arsenm, llvm-commits, kzhuravl

Differential Revision: http://reviews.llvm.org/D21038

llvm-svn: 272748