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/compiler/glsl/gl_nir_link_atomics.c

@@ -148,7 +148,7 @@ find_active_atomic_counters(struct gl_context *ctx,
 
       nir_shader *nir = sh->Program->nir;
 
-      nir_foreach_variable(var, &nir->uniforms) {
+      nir_foreach_uniform_variable(var, nir) {
          if (!glsl_contains_atomic(var->type))
             continue;