nir: Implement lowered SSBO atomic intrinsics

nir: Implement lowered SSBO atomic intrinsics

The original GLSL IR intrinsics have been lowered to an internal
version that accepts a block index and an offset instead of a
SSBO reference.

v2 (Connor):
  - Document the sources used by the atomic intrinsics.

Reviewed-by: Connor Abbott <connor.w.abbott@intel.com>
Reviewed-by: Kristian Høgsberg <krh@bitplanet.net>