[Dominators] Split SemiNCA into smaller functions

Summary:
This patch splits the SemiNCA algorithm into smaller functions. It also adds a new debug macro.

In order to perform incremental updates, we need to be able to refire SemiNCA on a subset of CFG nodes (determined by a DFS walk results). We also need to skip nodes that are not deep enough in a DomTree.

Reviewers: dberlin, davide, sanjoy, grosser

Reviewed By: dberlin, davide

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D35282

llvm-svn: 307950