Consider optimizing LeftSemi to emit output incrementally #22929

Is your feature request related to a problem or challenge?

Right now, LeftSemi consumes the entire probe side before emitting any output rows. This isn't fundamental: we could check which probe row flips the visited bitmap from 0 -> 1, and arrange for the output row to be produced at that point. This would enable better pipeline parallelism.

It would probably also improve partitioning behavior: LeftSemi + CollectLeft emits everything in a single output partition today, which isn't great.

Would need to check if this needs stronger memory ordering requirements when flipping bits than we can get away with today.

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