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

[Ryotaro Kasuga via llvm-commits]

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

Thanks for the reproducer. I'll take a closer look at the details. Just some initial quick comments.

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