[Dominators] Add NearestCommonDominator verification

Summary:
This patch adds another verification function for checking correctness of findNearestCommonDominator.
For every edge from U to V in the input graph, `NCD(U, V) == IDom(V) or V` -- the new function checks this condition.

Reviewers: dberlin, sanjoy, chandlerc

Reviewed By: dberlin

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D34575

llvm-svn: 306893