[llvm] [LoopInterchange] Prevent from undoing its own transformation (PR #127473)

[Ryotaro Kasuga via llvm-commits]

https://github.com/kasuga-fj commented:

Thanks for your review. I erased the off topic comments on this patch.

The problem is the current behavior of `isProfitablePerLoopCacheAnalysis` when `CC->getLoopCost(*OuterLoop) == CC->getLoopCost(*InnerLoop)`. It returns true if `InnerIndex < OuterIndex`, but otherwise returns nullopt, which may trigger to undo a past interchange.

https://github.com/llvm/llvm-project/pull/127473