[llvm] [DTU] fix dominator tree update eliding reachable nodes (PR #177683)

[Mircea Trofin via llvm-commits]

mtrofin wrote:

### Merge activity

* **Mar 4, 3:10 PM UTC**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.com/github/pr/llvm/llvm-project/177683).


https://github.com/llvm/llvm-project/pull/177683