AMDGPU/SI: add llvm.amdgcn.image.load/store[.mip] intrinsics

Summary:
These correspond to IMAGE_LOAD/STORE[_MIP] and are going to be used by Mesa
for the GL_ARB_shader_image_load_store extension.

IMAGE_LOAD is already matched by llvm.SI.image.load. That intrinsic has
a legacy name and pretends not to read memory.

Differential Revision: http://reviews.llvm.org/D17276

llvm-svn: 261224