[Dominators] Make IsPostDominator a template parameter

Summary:
DominatorTreeBase used to have IsPostDominators (bool) member to indicate if the tree is a dominator or a postdominator tree. This made it possible to switch between the two 'modes' at runtime, but it isn't used in practice anywhere.

This patch makes IsPostDominator a template argument. This way, it is easier to switch between different algorithms at compile-time based on this argument and design external utilities around it. It also makes it impossible to incidentally assign a postdominator tree to a dominator tree (and vice versa), and to further simplify template code in GenericDominatorTreeConstruction.

Reviewers: dberlin, sanjoy, davide, grosser

Reviewed By: dberlin

Subscribers: mzolotukhin, llvm-commits

Differential Revision: https://reviews.llvm.org/D35315

llvm-svn: 308040