nir/validate: Don't treat if-uses specially

We don't use the tag anywhere, so don't bother with it.

Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Suggested-by: Faith Ekstrand <faith@gfxstrand.net>
Reviewed-by: Faith Ekstrand <faith.ekstrand@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/22343>