[llvm] Revert "[VPlan] Remove unnecessary DomTreeUpdater flush (NFC)." (PR #144758)

[Arthur Eubanks via llvm-commits]

aeubanks wrote:

@fhahn I've been trying to come up with a reproducer, but it's in a ThinLTO build. am continuing to try this week. it might be an earlier pass not properly invalidating DT?

https://github.com/llvm/llvm-project/pull/144758