[Dominators] Implement incremental deletions

Summary:
This patch implements incremental edge deletions.

It also makes DominatorTreeBase store a pointer to the parent function. The parent function is needed to perform full rebuilts during some deletions, but it is also used to verify that inserted and deleted edges come from the same function.

Reviewers: dberlin, davide, grosser, sanjoy, brzycki

Reviewed By: dberlin

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D35342

llvm-svn: 308062