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

[Ryotaro Kasuga via llvm-commits]

kasuga-fj wrote:

> > * 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...
> 
> “Partially perfect loop” sounds good to me Also, would naming it something like sibling-loop-nests.ll make the intent clearer, or do you think that might be more confusing?

Either is fine with me, so please feel free to choose whichever you prefer. Also, please don't forget to rename the file.

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