ir3: Fix infinite loop in scheduler when splitting

ir3: Fix infinite loop in scheduler when splitting

When we go to split e.g. a p0.x producer, the only other instructions
ready to schedule are often only p0.x producers. It could happen that
they all have a lower priority than the split instruction. Then we would
immediately schedule the split instruction again, then again try to
schedule one of the other producers, be blocked, and split it, around
and around again, leading to an infinite loop. The following commit
triggered this with
dEQP-GLES3.functional.shaders.discard.dynamic_loop_always on a3xx.

Fixes: d2f4d33 ("freedreno/ir3: new pre-RA scheduler")
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6752>