nir: Add a nir_foreach_uniform_variable helper

This one's a bit more complex because it filters off only those
variables with mode == nir_var_uniform.  As such, it's not exactly a
drop-in replacement for nir_foreach_variable(var, &nir->uniforms).

Reviewed-by: Gert Wollny <gert.wollny@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5966>

src/broadcom/compiler/vir.c

@@ -570,7 +570,7 @@ v3d_lower_nir(struct v3d_compile *c)
         }
 
         /* CS textures may not have return_size reflecting the shadow state. */
-        nir_foreach_variable(var, &c->s->uniforms) {
+        nir_foreach_uniform_variable(var, c->s) {
                 const struct glsl_type *type = glsl_without_array(var->type);
                 unsigned array_len = MAX2(glsl_get_length(var->type), 1);