[Dominators] Add parent and sibling property verification (non-hacky)

Summary:
This patch adds an additional level of verification - it checks parent and sibling properties of a tree. By definition, every tree with these two properties is a dominator tree.

It is possible to run those check by running llvm with `-verify-dom-info=1`.

Bootstrapping clang and building the llvm test suite with this option enabled doesn't yield any errors.

Reviewers: dberlin, sanjoy, chandlerc

Reviewed By: dberlin

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D34482

llvm-svn: 306711