ir3/sched: Speed up live_effect

ir3/sched: Speed up live_effect

If we've identified another use that isn't scheduled yet, we can break
right away rather than iterating through all the other uses. While this
could be optimized further, this simple change makes
dEQP-VK.subgroups.ballot_broadcast.compute.subgroupbroadcast_ivec4 go
from 40 seconds to 1.9 seconds on a release build according to my
unscientific testing.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/11613>