[llvm] [GenericDomTreeConstruction] Use 0-based DFS numbering and a trivial InfoRec (PR #207524)

[Fangrui Song via llvm-commits]

MaskRay wrote:

Reduced the inline capacity of NumToNode/NodeInfos to 32. 
https://llvm-compile-time-tracker.com/compare.php?from=3dc58a6e2c5ad5254883e6a5c43e2e6ea46c42af&to=1bd2dedd34962b737deb0c6c6d3e67a5d6517c46&stat=instructions:u shows no significance, apart from stage1-O3 improvement likely due to increasing the capacity for `ReverseChildren`.

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