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

[David Blaikie via llvm-commits]

dwblaikie wrote:

> 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. 

Ah, cool - thanks for the explanation!

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