[ADCE][Dominators] Teach ADCE to preserve dominators

Summary:
This patch teaches ADCE to preserve both DominatorTrees and PostDominatorTrees.

I didn't notice any performance impact when bootstrapping clang with this patch.

Reviewers: dberlin, chandlerc, sanjoy, davide, grosser, brzycki

Reviewed By: davide

Subscribers: grandinj, zhendongsu, llvm-commits, david2050

Differential Revision: https://reviews.llvm.org/D35869

llvm-svn: 311039