[ADCE][Dominators] Reapply: Teach ADCE to preserve dominators

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

This is reapplies the original patch r311057 that was reverted in r311381.
The previous version wasn't using the batch update api for updating dominators,
which in vary rare cases caused assertion failures.

This also fixes PR34258.

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: 311467