[llvm] [LoopInterchange] Use UTC as much as possible (NFC) (PR #202096)

[Ryotaro Kasuga via llvm-commits]

kasuga-fj wrote:

To the reviewers: For this PR, rather than asking you to review the individual file changes, I'd primarily like to hear your opinion on whether this overall approach is reasonable. As for the file changes themselves, I had some AI assistance, but I also double-checked them, so I believe they should be fine.
As I mentioned in the PR description, this approach does have a downside in that the tests become less human-readable. That said, I personally still find this to be the better experience.
Also, for some of the test cases (e.g., `large-nested{4,6}d.ll`), I've left them as they are, since in my understanding it's important for those tests to be able to see where exactly the transformation stucks.

https://github.com/llvm/llvm-project/pull/202096