nir: Be sure to fix deref modes after demoting shader i/o vars to global.

Fixes assertion failures when calling nir_remove_unused_varyings() or
nir_remove_unused_io_vars().

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>

src/compiler/nir/nir_linking_helpers.c

@@ -126,6 +126,9 @@ remove_unused_io_vars(nir_shader *shader, struct exec_list *var_list,
       }
    }
 
+   if (progress)
+      nir_fixup_deref_modes(shader);
+
    return progress;
 }