[llvm] [CycleInfo] Store blocks using Euler tour representation (PR #208614)

[Alexis Engelke via llvm-commits]

https://github.com/aengelke approved this pull request.

LGTM. But please wait until after branching before merging. This brings CycleInfo into something that is closer to what I have in mind, thank you for working on this.

On CI pointer: instead of having Cycle methods taking a CI pointer, I would propose a different design: Cycle becomes an integer (or a struct wrapping an integer) and all methods of Cycle get moved to CycleInfo taking the Cycle as argument. This permits inline implementations of functions (in contrast to the current awkward out-of-line contains/blocks_{begin,end}) and should also ease any future changes to the data representation.

https://github.com/llvm/llvm-project/pull/208614