[llvm] [GenericDomTreeConstruction] Switch to 0-based DFS numbering (PR #207524)

[Fangrui Song via llvm-commits]

MaskRay wrote:

I am experimenting with another optimization that keeps `NumToNote/NumToInfo/IDoms` 0-based and `DFSNum` 1-based, while extracting `ReverseChildren` to make `InfoPtr` trivial. This will be a follow-up patch.

https://github.com/llvm/llvm-project/pull/207524