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>
diff --git a/src/panfrost/bifrost/compiler.h b/src/panfrost/bifrost/compiler.h
index 16f44ff..3b54c48 100644
--- a/src/panfrost/bifrost/compiler.h
+++ b/src/panfrost/bifrost/compiler.h
@@ -60,7 +60,7 @@
         BI_FMA,
         BI_FMOV,
         BI_FREXP,
-        BI_ISUB,
+        BI_IMATH,
         BI_LOAD,
         BI_LOAD_UNIFORM,
         BI_LOAD_ATTR,