[GVN] introduce GVNOptions to control GVN pass behavior

There are a few global (cl::opt) controls that enable optional
behavior in GVN. Introduce GVNOptions that provide corresponding
per-pass instance controls.

That will allow to use GVN multiple times in pipeline each time
with different settings.

Reviewers: asbirlea, rnk, reames, skatkov, fhahn
Reviewed By: fhahn

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D72732