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

[Aleksandr Popov via llvm-commits]

aleks-tmb wrote:

> In particular I'd like to understand why neither the SCEV VH callback invalidation nor the existing forgetBlockAndLoopDispositions call cover this.

It seems the reason why forgetBlockAndLoopDispositions doesn't cover this case is that `%length.lcssa.ph` doesn't have an existing SCEV, since the instruction was just added by the previous LICM pass.

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