[llvm] [DominanceFrontier] Support post-dominators on graphs with single root (PR #179336)

[Florian Hahn via llvm-commits]

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

LGTM, thanks.

The updated note at the top of the file makes sense to me, and this is not a concern for the new use case.

Probably worth waiting a day or two with landing, in case @kuhar or anybody else has additional thoughts/concerns

https://github.com/llvm/llvm-project/pull/179336