[ADCE] Refactor anticipating new functionality (NFC)

Summary:
This is the first refactoring before adding new functionality.
Add a class wrapper for the functions and container for
state associated with the transformation.

No functional change

Reviewers: majnemer, nadav, mehdi_amini

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D23065

llvm-svn: 277565