AMDGPU/SI: Add llvm.amdgcn.buffer.atomic.* intrinsics

Summary:
These intrinsics expose the BUFFER_ATOMIC_* instructions and will be used
by Mesa to implement atomics with buffer semantics. The intrinsic interface
matches that of buffer.load.format and buffer.store.format, except that the
GLC bit is not exposed (it is automatically deduced based on whether the
return value is used).

The change of hasSideEffects is required for TableGen to accept the pattern
that matches the intrinsic.

Reviewers: tstellarAMD, arsenm

Subscribers: arsenm, rivanvx, llvm-commits

Differential Revision: http://reviews.llvm.org/D18151

llvm-svn: 263791