[llvm] [LoopInterchange] Skip legality check if surrounding loops already guarantee dependency (NFC) (PR #139254)

[Ryotaro Kasuga via llvm-commits]

kasuga-fj wrote:

> Can you make the title more concrete than "certain cases"? It's not an early exit either
>
> Suggestion: Skip legality check if surronding loops already guarantee dependency

Fixed, thanks!

https://github.com/llvm/llvm-project/pull/139254