Migrate SimplifyLibCalls to new OptimizationRemarkEmitter

Summary:
This changes SimplifyLibCalls to use the new OptimizationRemarkEmitter
API.

In fact, as SimplifyLibCalls is only ever called via InstCombine,
(as far as I can tell) the OptimizationRemarkEmitter is added there,
and then passed through to SimplifyLibCalls later.

I have avoided changing any remark text.

This closes PR33787

Patch by Sam Elliott!

Reviewers: anemet, davide

Reviewed By: anemet

Subscribers: davide, mehdi_amini, eraman, fhahn, llvm-commits

Differential Revision: https://reviews.llvm.org/D35608

llvm-svn: 309158