[Dominators] Keep tree level in DomTreeNode and use it to find NCD and answer dominance queries

Summary:
This patch makes DomTreeNodes keep their level (depth) in the DomTree. By having this information always available, it is possible to speedup and simplify findNearestCommonDominator and certain dominance queries.

In the future, level information will be also needed to perform incremental updates.

My testing doesn't show any noticeable performance differences after applying this patch. There may be some improvements when other passes are thought to use the level information.

Reviewers: dberlin, sanjoy, chandlerc, grosser

Reviewed By: dberlin

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D34548

llvm-svn: 306892