pan/bi: Add dummy carry/borrow argument for iadd/isub

pan/bi: Add dummy carry/borrow argument for iadd/isub

On FMA, a carry/borrow is required for iaddc/isubb (whereas the ADD
counterparts don't support carrying/borrowing). The trick is to model
this with an extra dummy (ZERO) argument which is free to encode on FMA,
and in the scheduler, "demote" to the non-carried versions if we want to
schedule to ADD.

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6749>