[Dominators] Move root-finding out of DomTreeBase and simplify it

[kuhar]

Summary

This patch moves root-finding logic from DominatorTreeBase to GenericDomTreeConstruction.h.
It makes the behavior simpler and more consistent by always adding a virtual root to PostDominatorTrees.

[kuhar]

Update the comment about virtual roots.

[kuhar]

Ping. I have patches depending on this one.

[hiraditya]

include/llvm/Support/GenericDomTree.h

216

s/NodeT */NodePtr