When discarding a fragment in Metal, it will not be demoted to helper. At least for Apple Silicon M1 and M2. Call nir_lower_is_helper_invocation to work around this. Reviewed-by: Arcady Goldmints-Orlov <arcady@lunarg.com> Signed-off-by: Aitor Camacho <aitor@lunarg.com> Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/38590>