[llvm] [JumpThreading] Don't phi translate past loop phi (PR #70664)

[Nikita Popov via llvm-commits]

nikic wrote:

The other problem here is that LoopHeaders may not be up-to-date. As it was previously only used for heuristics, using it for correctness now may be a bit iffy.

https://github.com/llvm/llvm-project/pull/70664