nir: Move compute system value lowering to a separate pass

The actual variable -> intrinsic lowering stays where it is, but
ops which convert one intrinsic to be implemented in terms of
another have moved.

Reviewed-by: Karol Herbst <kherbst@redhat.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5891>

src/intel/compiler/brw_nir.c

@@ -709,6 +709,7 @@ brw_preprocess_nir(const struct brw_compiler *compiler, nir_shader *nir,
    }
 
    OPT(nir_lower_system_values);
+   OPT(nir_lower_compute_system_values);
 
    const nir_lower_subgroups_options subgroups_options = {
       .ballot_bit_size = 32,