agx: Add p_split pseudoinstruction

Easier on RA for extracts.

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16268>

src/asahi/compiler/agx_opcodes.py

@@ -253,4 +253,5 @@ op("and", _, srcs = 2)
 op("or", _, srcs = 2)
 
 op("p_combine", _, srcs = 4)
+op("p_split", _, srcs = 1, dests = 4)
 op("p_extract", _, srcs = 1, imms = [COMPONENT])