nir/lower_io_passes: call nir_opt_undef to eliminate undef output stores

If we do it here, we won't have to call nir_recompute_io_bases later again.

Acked-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/38598>

src/compiler/nir/nir_lower_io.c

@@ -1274,6 +1274,12 @@ nir_lower_io_passes(nir_shader *nir, bool renumber_vs_inputs)
    NIR_PASS(_, nir, nir_opt_dce);
    NIR_PASS(_, nir, nir_remove_dead_variables, nir_var_function_temp, NULL);
 
+   /* Output stores can have undef values. Eliminate those before
+    * nir_recompute_io_bases. This happens with separate shaders, which are
+    * usually not optimized further after this.
+    */
+   NIR_PASS(_, nir, nir_opt_undef);
+
    /* If IO is lowered before var->data.driver_location is assigned, driver
     * locations are all 0, which means IO bases are all 0. It's not necessary
     * to set driver_location before lowering IO because the only thing that