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

[Sjoerd Meijer via llvm-commits]

sjoerdmeijer wrote:

> 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.

I see what you're saying about tests where transformations don't trigger, but in this case I think the motivating examples are useful in our discussions. We have so many moving parts, and now we can use these real-examples to test our ideas and share results. So, I will go ahead and merge this my tomorrow morning.

If these tests become a pain or a burden, it is easy to change or remove them, but to be honest, I don't expect it. And for now, I would actually be curious to know if and when they need changing, that would be good to know. But anyway, let's see how it goes. 

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