AMDGPU/SI: add llvm.amdgcn.image.atomic.* intrinsics

These correspond to IMAGE_ATOMIC_* and are going to be used by Mesa for the
GL_ARB_shader_image_load_store extension.

Initial change by Nicolai H.hnle

Differential Revision: http://reviews.llvm.org/D17401

llvm-svn: 262701