AMDGPU: mark llvm.amdgcn.image.atomic.* as a source of divergence

Summary:
When multiple threads perform an atomic op with the same arguments, they
will usually see different return values.

Reviewers: arsenm, tstellarAMD

Subscribers: arsenm, llvm-commits

Differential Revision: http://reviews.llvm.org/D18101

llvm-svn: 263440