[llvm] Add test for imperfect loop nest in LoopInterchange (PR #201507)

[Ryotaro Kasuga via llvm-commits]

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

A couple of nit comments:
- For the PR title: by convention, we prefix it with the pass name. Also, for non-functional changes like this one, we often add "(NFC)" at the end. So I'd suggest changing the title to something like "[LoopInterchange] Add test for imperfect loop nest (NFC)".
- I just felt that the term "imperfect" might be confusing, because there are several kinds of imperfect loop nests, not only this one. Maybe something like "partially perfect loop" would be clearer? That said, I don't have a better idea myself...

https://github.com/llvm/llvm-project/pull/201507