Introduce DebugCounter into ConstProp pass

Summary:
This patch introduces DebugCounter into ConstProp pass at per-transformation level.

It will provide an option to skip first n or stop after n transformations for the whole ConstProp pass.

This will make debug easier for the pass, also providing chance to do transformation level bisecting.

Reviewers: davide, fhahn

Reviewed By: fhahn

Subscribers: llozano, george.burgess.iv, llvm-commits

Differential Revision: https://reviews.llvm.org/D50094

llvm-svn: 346720