pan/bi: Introduce writemasks

I feel so dirty. But this will let the IR be a lot more flexible seeing
as we really are vector in a certain sense (I/O, small types)

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4139>
3 files changed