pan/bi: Eliminate writemasks in the IR

Since the hardware doesn't support them, they're a burden to deal with,
so let's ensure we never get to a place where we would need to at all.

Disables COMBINE lowering for now.

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4766>
10 files changed