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

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.

Reviewers: dberlin, davide, grosser, sanjoy

Reviewed By: dberlin

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D35597

llvm-svn: 309146