[VPlan] Use DomTreeUpdater to automatically update DT for vector loop. (#92525)

Use DTU to queue DominatorTree updates directly when connecting basic
blocks during VPlan execution.

This simplifies DT updates and will also automatically allow updating
the DT for the VPlan-native path as additional benefit in a follow-up.

PR: https://github.com/llvm/llvm-project/pull/92525