[Dominators] Rearrange access specifiers in DominatorTreeBase

Summary:
This patch makes DominatorTreeBase more readable by putting most important members on top of the class.

Before, the class looked like that: private -> protected (including data members) -> public -> protected.
The patch changes it to: protected (data members only) -> public -> protected -> public.

Reviewers: dberlin, sanjoy, chandlerc

Reviewed By: dberlin

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D34527

llvm-svn: 306714