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

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darkbuck wrote:

> I thought JumpThreading did this, why is SImplifyCFG here?

Compared to jump-threading, SimplifyCFG also does limited threading. For example, `FoldCondBranchOnValueKnownInPredecessor` also threads basic blocks, as explained in this commit message. However, I have no idea why we have duplicated functionality here and in JumpThreading. Cleaning them up in a dedicated one and, if needed, invoking them from SimplifyCFG should be addressed in a dedicated PR as more tests could be changed accordingly.

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