This commit adds the `JS::ubi::DominatorTree` class. It uses the simple,
engineered algorithm for finding immediate dominators described in "A Simple,
Fast Dominance Algorithm" by Cooper et al[0]. This commit does not expose
dominator trees to JS, or provide any means of walking the dominator tree. These
things are reserved for future changesets.
[0]: http://www.cs.rice.edu/~keith/EMBED/dom.pdf