[llvm] [DomTree] Fix root attachment in runDFS() (PR #73148)

[Nikita Popov via llvm-commits]

nikic wrote:

> Does this need test coverage? Or is the assert essentially adding test coverage? (eg: would adding the assert have fired at ToT without the other changes?)

Yes, the assert would fire without the changes. The change is intended to be NFC in terms of externally observable behavior, it only fixes internal state that is incorrect in a way that ends up not mattering with the current implementation. I ran into it when implementing https://github.com/llvm/llvm-project/pull/73097.

https://github.com/llvm/llvm-project/pull/73148