[Dominators] Remove the NCA check

Summary:
This patch removes the `verifyNCD` check.

The reason for this is that the other checks are sufficient to prove or disprove correctness of any DominatorTree, and that `verifyNCD` doesn't provide (in my option) better error messages then the other ones.
Additionally, this should give a (small) improvement to the total verification time, as the check is O(n), and checking the sibling property takes O(n^3).

Reviewers: dberlin, grosser, davide, brzycki

Reviewed By: brzycki

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D38802

llvm-svn: 315790