[Dominators] Use Semi-NCA instead of SLT to calculate dominators

Summary:
This patch makes GenericDomTreeConstruction use the Semi-NCA algorithm instead of Simple Lengauer-Tarjan.

As described in `RFC: Dynamic dominators`, Semi-NCA offers slightly better performance than SLT. What's more important, it can be extended to perform incremental updates on already constructed dominator trees.

The patch passes check-all, llvm test suite and is able to boostrap clang. I also wasn't able to observe any compilation time regressions.

Reviewers: sanjoy, dberlin, chandlerc, grosser

Reviewed By: dberlin

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D34258

llvm-svn: 306437