nir: Make image load/store intrinsics variable-width

nir: Make image load/store intrinsics variable-width

Instead of requiring 4 components, this allows them to potentially use
fewer.  Both the SPIR-V and GLSL paths still generate vec4 intrinsics so
drivers which assume 4 components should be safe.  However, we want to
be able to shrink them for i965.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>