[llvm] [LoopInterchange] Motivating example for interchange. NFC. (PR #171631)

[Ryotaro Kasuga via llvm-commits]

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

I'm not sure if it is a good idea to add motivating examples where interchange still cannot be applied to regression tests (if the remark outputs change frequently due to minor modifications to LoopInterchange, it might be burdensome), but I don't have any strong objections.

https://github.com/llvm/llvm-project/pull/171631