[llvm] [LoopInfo] Derive Loop::contains(BlockT*) from the block-to-loop map (PR #207613)

[Fangrui Song via llvm-commits]

MaskRay wrote:

> Would it make sense to implement Loop::contains() with a lazily computed DFS numbering like we do for DT?

`[LoopInfo] Answer contains() in O(1) with Euler-tour loop numbering` has minor regression
https://llvm-compile-time-tracker.com/compare.php?from=e1386d66558c3a326a19ed6624092402b81a398f&to=0770be2660db2caf46b2278c50a0a91ebe42d269&stat=instructions:u

With 101 `addBasicBlockToLoop` we cannot change the block layout.

https://github.com/llvm/llvm-project/pull/207613