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/gallium/auxiliary/nir/nir_draw_helpers.c

@@ -123,7 +123,7 @@ nir_lower_pstipple_fs(struct nir_shader *shader,
       return;
 
    int binding = 0;
-   nir_foreach_variable(var, &shader->uniforms) {
+   nir_foreach_uniform_variable(var, shader) {
       if (glsl_type_is_sampler(var->type)) {
          if (var->data.binding >= binding)
             binding = var->data.binding + 1;