[DomTree] Extend update API to allow a post CFG view.

Extend the `applyUpdates` in DominatorTree to allow a post CFG view,
different from the current CFG.
This patch implements the functionality of updating an already up to
date DT, to the desired PostCFGView.
Combining a set of updates towards an up to date DT and a PostCFGView is
not yet supported.

Differential Revision: https://reviews.llvm.org/D85472