asahi: Run nir_lower_fragcolor during preprocessing

asahi: Run nir_lower_fragcolor during preprocessing

This pass needs to run early (because it depends on early I/O), but it doesn't
actually need the shader key. Why not? If we overestimate the number of render
targets, extra store_output intrinsics will be generated, but they will be
deleted by AGX tilebuffer lowering later.

Note we'll probably want something smarter than this for fragment epilogues in
the future to avoid piles of unnecessary moves.

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21065>