pan/bi: Rewrite aligned vectors as well

pan/bi: Rewrite aligned vectors as well

This still isn't optimal, but it handles another common case where we
have a vector "prefix" and can rewrite directly. The last case is one
where writemasks and such would start coming into play and - for the
moment - is not worth the hike in complexity.

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4288>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4288>