[Dominators] Implement incremental insertions

Summary:
This patch introduces incremental edge insertions based on the Depth Based Search algorithm.

Insertions should work for both dominators and postdominators.

Reviewers: dberlin, grosser, davide, sanjoy, brzycki

Reviewed By: dberlin

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D35341

llvm-svn: 308054