[Dominators] Move SemiNCAInfo and helper functions out of DominatorTreeBase

Summary:
This moves SemiNCAInfo from DeminatorTreeBase to GenericDomTreeConstruction. It also put helper functions used during tree constructions in the same file.

The point of this change is to further clean up DominatorTreeBase and make it easier to construct and verify (in future patches).

Reviewers: dberlin, sanjoy, chandlerc

Reviewed By: dberlin

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D34420

llvm-svn: 306576