[Dominators] Simplify templates

Summary: DominatorTreeBase and related classes used overcomplicated template machinery. This patch simplifies them and gets rid of DominatorTreeBaseTraits and DominatorTreeBaseByTraits, which weren't actually used outside the DomTree construction.

Reviewers: dberlin, sanjoy, davide, grosser

Reviewed By: dberlin, davide, grosser

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D35285

llvm-svn: 307953