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

Summary: This partially reverts commit r311057 since it breaks ADCE.  See PR34258.

Reviewers: kuhar

Subscribers: mcrosier, david2050, llvm-commits

Differential Revision: https://reviews.llvm.org/D36979

llvm-svn: 311381