Clean up the logic and move it to functions that work with prog_data
attributes to select the right SIMD. This shouldn't change any
behavior compared to the original.
Having it extracted will allow reuse by Task/Mesh and make it easier
to write tests.
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/13249>
