[LoopIdiom] Basic OptimizationRemarkEmitter handling

Summary:
I'm adding ORE to memset/memcpy formation, with tests,
but mainly this is split off from D61144.

Reviewers: reames, anemet, thegameg, craig.topper

Reviewed By: thegameg

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D62631

llvm-svn: 362092