[llvm] [Uniformity] Fixed control-div early stop (PR #138806)

Tue May 6 22:50:54 PDT 2025

https://github.com/jgu222 created https://github.com/llvm/llvm-project/pull/138806

Control-divergence finds joins by propagating labels from the divergent control branch. The code checking if the early stop is reached is not correct.

This change fixes this issue by checking if a join is reached. The propagation is still the same in which propagation starts by adding successors of the divergent block into a set first and then propagate one from the set in the toplogical order.
If there is only one block in the set, then this block must be a join block, and thus propagation stops.

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