Commit 28311f9d02 moved ufind_msb lowering to NIR and started emitting
uclz. Unfortunately, the vec4 backend never actually implemented uclz.
It's trivial to do. Now it does.
Fixes: 28311f9d02 ("nir: intel/compiler: Move ufind_msb lowering to NIR")
Reviewed-by: Faith Ekstrand <faith.ekstrand@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21974>