pan/bi: Ingest vecN directly (again)

Last time, I swear. We still generate writemasks but SSA-like ones and
do the lowering ourselves.

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