radv: Move preparation and fixup to separate loops in varying optimization.

This is to stop calling nir_shader_gather_info repeatedly for
some stages, and also as a pre-requisite to the work in the next commits.

No Fossil DB changes.

Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/33880>
1 file changed