iris: Reference the shader variant for last_vue_map as well

We call update_last_vue_map after updating the shaders, which compares
the new and old VUE maps.  Except...updating the shaders may have
dropped the last reference to the variant that ice->shaders.last_vue_map
belonged to, leading to a classic use-after-free.

Fix this by taking a reference to the variant for the last VUE stage,
so it stays around until we're done with it.

Fixes: 1afed51445 ("iris: Store a list of shader variants in the shader itself")
Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/4311
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9143>

src/gallium/drivers/iris/iris_program_cache.c

@@ -281,6 +281,7 @@ iris_destroy_program_cache(struct iris_context *ice)
    for (int i = 0; i < MESA_SHADER_STAGES; i++) {
       iris_shader_variant_reference(&ice->shaders.prog[i], NULL);
    }
+   iris_shader_variant_reference(&ice->shaders.last_vue_shader, NULL);
 
    hash_table_foreach(ice->shaders.cache, entry) {
       struct iris_compiled_shader *shader = entry->data;