[llvm] [polly] [LoopInfo] Add an opaque LoopRef handle and stop exposing loop containers (PR #210653)

[Alexis Engelke via llvm-commits]

aengelke wrote:

I think the PR should be split:

- Improved updating facilities. These seem ok.
- Moving some things to LoopInfo. Doable, although no benefit?
- LoopRef. What's the long-term plan here? I don't think it's feasible to do deeper changes or improvements to LoopInfo -- the current design is depended upon in so many places. This might just end up as a lot of code churn for little (which exact?) benefit and another half-finished migration?

https://github.com/llvm/llvm-project/pull/210653