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

[Fangrui Song via llvm-commits]

MaskRay wrote:

> Yeah, I'm only concerned about the non-zero default DFSNum in InfoRec, everything else is fine. Re ReverseChildren: [this](https://github.com/llvm/llvm-project/compare/main...aengelke:perf/domtree-preds) is what I had in mind. _edit:_ [c-t-t results](https://llvm-compile-time-tracker.com/compare.php?from=725051d69b733910226687a99dacc6ffc8805aab&to=4905a930f146871cf88aebf54fba11bb79715f72&stat=instructions:u) show a slight (stage2-O3 -0.06%) improvement.

Nice! This is the Chain Forward Star representation of the reverse graph.

I've also tried Compressed Sparse Row. While there is also improvement, there is more code, so inferior.
https://llvm-compile-time-tracker.com/compare.php?from=5b99dcd1f6ea1871e17af1e53049f518336e0995&to=3347744077adad9022df55f7453f045bb2a3b4b7&stat=instructions:u

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