pan/bi: Add isub op

Can't be a regular ADD since there's no negate modifier for integers
(it's a different opcode entirely).

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