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

[Fangrui Song via llvm-commits]

MaskRay wrote:

As a follow-up, we can delete `LoopInterchangeTransform::restructureLoops` in favor of `recompute()`

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