[llvm] [LoopInterchange] Relax the legality check to accept more patterns (PR #118267)

[Ryotaro Kasuga via llvm-commits]

kasuga-fj wrote:

> I don't fully understand why normalize becomes unnecessary after this patch. Can you please elaborate?

I believe the normalization is called because `isLegalToInterChangeLoops` assumes non-negative direction vectors. If the logic of this patch is correct, I think this assumption becomes unnecessary.

I also realized that the removal of `S` affects the logic of my change (I was mistaken in my previous comment). I will fix the code.

https://github.com/llvm/llvm-project/pull/118267