[Dominators] Improve reachability verification

Summary:
This patch improves verification by making `verifyReachablility` look for CFG not found in the DomTree.
It also makes the verification work with postdominators by handling virtual root.

Reviewers: dberlin, davide, grosser, sanjoy

Reviewed By: dberlin

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D35279

llvm-svn: 307936