[Dominators] Always recalculate postdominators when update yields different roots

Summary:
This patch makes postdominators always recalculate the tree when an update causes to change the tree roots.
As @dmgreen noticed in [[ https://reviews.llvm.org/D41298 | D41298 ]], the previous implementation was not conservative enough and it was possible to end up with a PostDomTree that was different than a freshly computed one.
The patch also compares postdominators with a freshly computed tree at the end of full verification to make sure we don't hit similar issues in the future.

This should (ideally) be also backported to 6.0 before the release, although I don't have any reports of this causing an observable error. It should be safe to do it even if it's late in the release, as the change only makes the current behavior more conservative.

Reviewers: dmgreen, dberlin, davide, brzycki, grosser

Reviewed By: brzycki, grosser

Subscribers: llvm-commits, dmgreen

Differential Revision: https://reviews.llvm.org/D43140

llvm-svn: 324962