AMDGPU: Implement {BUFFER,FLAT}_ATOMIC_CMPSWAP{,_X2}

Summary:
Implement BUFFER_ATOMIC_CMPSWAP{,_X2} instructions on all GCN targets, and FLAT_ATOMIC_CMPSWAP{,_X2} on CI+.

32-bit instruction variants tested manually on Kabini and Bonaire. Tests and parts of code provided by Jan Veselý.

Patch by: Vedran Miletić

Reviewers: arsenm, tstellarAMD, nhaehnle

Subscribers: jvesely, scchan, kanarayan, arsenm

Differential Revision: http://reviews.llvm.org/D17280

llvm-svn: 265170