[llvm] [SimplifyCFG] Skip threading if the target may have divergent branches (PR #100185)

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darkbuck wrote:

> Maybe all of this can be moved to Transforms/Scalar/JumpThreading.cpp ? simplifycfg is doing too many things which could be better suited as separate passes.

Yeah, SimplifyCFG has few limited duplications to that dedicated pass. But it should be addressed in another dedicated PR as it may have measurable impacts compared to this one.

https://github.com/llvm/llvm-project/pull/100185