nir/link_opt_varyings: Make it participate in NIR_DEBUG=print.

It's a pass with major effects on shaders, and it's otherwise weird to see
your varying disappear between two passes that shouldn't affect them.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/37313>

src/compiler/nir/nir_linking_helpers.c

@@ -1434,6 +1434,15 @@ nir_link_opt_varyings(nir_shader *producer, nir_shader *consumer)
 
    _mesa_hash_table_destroy(varying_values, NULL);
 
+   if (should_print_nir(producer)) {
+      printf("nir_link_opt_varyings\n");
+      nir_print_shader(producer, stdout);
+   }
+   if (should_print_nir(consumer)) {
+      printf("nir_link_opt_varyings\n");
+      nir_print_shader(consumer, stdout);
+   }
+
    return progress;
 }