pan/bi: Rework FAU lowering

pan/bi: Rework FAU lowering

Move and reshape bi_lower_fau to bi_schedule.c. This generalizes the
pass for FAU reads, allowing copyprop to work with FAU without problems.

The pass must run immediately before scheduling. Its post-conditions are
directly specified as the scheduler's pre-conditions. It momentarily
will depend on internal scheduler predicates. It is, for all intents and
purposes, part of the scheduler. Keep it all together.

Finally, adjust the 0 handling to avoid a move at the expense of
constrained scheduling of something like `FADD.v2f16.clamp_0_1 u0, #0`

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8973>