[llvm] [LoopInfo] Add recompute() and use it in SimpleLoopUnswitch (PR #213514)

[Alexis Engelke via llvm-commits]

https://github.com/aengelke commented:

Tentative LGTM. Are there relevant pathological cases where we have worse perf now?

Maybe we should wait until @nikic is back from vacation. IIRC, when we briefly discussed the "recompute instead of update" direction at EuroLLVM, he wasn't entirely convinced.

https://github.com/llvm/llvm-project/pull/213514