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

[Alexis Engelke via llvm-commits]

aengelke wrote:

Yeah, I'm only concerned about the non-zero default DFSNum in InfoRec, everything else is fine. Re ReverseChildren: [this](resizeImpl) is what I had in mind. c-t-t results are pending.

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