nir: make lower_sample_tex_compare a common pass

This pass was originally written for d3d12, but is useful for hardware
that lacks sample compare support like some etnaviv GPU models.

Also rename the lowering pass and some surrounding code to
nir_lower_tex_shadow as suggested by Emma.

I'd like to use the pass that's already in tree.

Signed-off-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Reviewed-by: Emma Anholt <emma@anholt.net>
Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/14308>

src/compiler/nir/meson.build

@@ -193,6 +193,7 @@ files_libnir = files(
   'nir_lower_ssbo.c',
   'nir_lower_subgroups.c',
   'nir_lower_system_values.c',
+  'nir_lower_tex_shadow.c',
   'nir_lower_tex.c',
   'nir_lower_texcoord_replace.c',
   'nir_lower_to_source_mods.c',