panfrost: Preprocess shaders in the driver

panfrost: Preprocess shaders in the driver

This is a flag-day change to how we compile. We split preprocessing NIR into a
separate step from compiling, giving the driver a chance to apply its own
lowerings on the preprocessed NIR before the final optimization loop. During
that time, the different producers of NIR (panfrost, panvk, blend shaders, blit
shaders...) will be able to (differently) lower system values.

Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20906>