[llvm] [SCEV] Cache collected loop guards. NFCI (PR #116947)

[Nikita Popov via llvm-commits]

https://github.com/nikic commented:

This is going to run up against our best friend: SCEV invalidation. I think invalidation for loop guards would be pretty tricky. It's easy to handle for loop invalidation, but we also need to invalidate if any of the SCEVs in the rewrite map are invalidated, and possibly also for certain control flow changes.

https://github.com/llvm/llvm-project/pull/116947