nir/spirv: Add support for separate samplers and textures

nir/spirv: Add support for separate samplers and textures

This gets tricky in a few places because we have to pass vtn_sampled_image
values through OpAccessChain, but it works ok.  At some point, it probably
needs to be cleaned up but it doesn't occur to me exactly how to do that at
the moment.  We'll see how this approach goes.