pan/bi: Rename BI_ISUB to BI_IMATH

We'll use this for iadd, etc too which share similar characteristics.

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4890>
6 files changed