[llvm] [LoopFlatten] Invalidate SCEV on each widening (PR #211819)

[Ryotaro Kasuga via llvm-commits]

https://github.com/kasuga-fj approved this pull request.

LGTM, but please wait for a while (maybe about a week?) to see if others have opinions.

https://github.com/llvm/llvm-project/pull/211819