agx: Dynamically size split instruction

This is more flexible.

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/18804>

src/asahi/compiler/agx_opcodes.py

@@ -266,7 +266,7 @@ op("or", _, srcs = 2)
 op("logical_end", _, dests = 0, srcs = 0, can_eliminate = False)
 
 op("combine", _, srcs = VARIABLE)
-op("split", _, srcs = 1, dests = 4)
+op("split", _, srcs = 1, dests = VARIABLE)
 op("phi", _, srcs = VARIABLE)
 
 op("unit_test", _, dests = 0, srcs = 1, can_eliminate = False)