[Dominators] Move helper functions into SemiNCAInfo

Summary: Helper functions (DFSPass, ReverseDFSPass, Eval) need SemiNCAInfo anyway, so it's simpler to have them there as member functions. This also makes them simpler by removing template boilerplate.

Reviewers: dberlin, sanjoy, chandlerc

Reviewed By: dberlin

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D34427

llvm-svn: 306579