nir: Add pass to lower phi precision

In addition to register pressure benefits from getting more fp16/int16,
this avoids i2imp's from standing in the way of loop unrolling.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Connor Abbott <cwabbott0@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/11545>

src/compiler/nir/meson.build

@@ -234,6 +234,7 @@ files_libnir = files(
   'nir_opt_move_discards_to_top.c',
   'nir_opt_offsets.c',
   'nir_opt_peephole_select.c',
+  'nir_opt_phi_precision.c',
   'nir_opt_rematerialize_compares.c',
   'nir_opt_remove_phis.c',
   'nir_opt_shrink_vectors.c',