nir: Add a preamble optimization pass

This pass tries to move computations that are uniform for the entire
draw to the preamble. There's also an API for backends to insert their
own instructions into the preamble, for porting existing UBO pushing
passes.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/13148>

src/compiler/nir/meson.build

@@ -244,6 +244,7 @@ files_libnir = files(
   'nir_opt_offsets.c',
   'nir_opt_peephole_select.c',
   'nir_opt_phi_precision.c',
+  'nir_opt_preamble.c',
   'nir_opt_ray_queries.c',
   'nir_opt_rematerialize_compares.c',
   'nir_opt_remove_phis.c',