[llvm] [LoopSimplifyCFG] Fix SCEV invalidation after removing dead exit (PR #127536)

[Nikita Popov via llvm-commits]

https://github.com/nikic commented:

As commented at https://github.com/llvm/llvm-project/issues/127534#issuecomment-2668711654, I'm not totally sure this fixes the root problem. But at the same time I don't think the extra forgetValue() will hurt either, so I wouldn't mind landing this.

https://github.com/llvm/llvm-project/pull/127536