[NewPassManager] Add tuning option: SLPVectorization [clang-change]

Summary:
NewPassManager is not using CodeGenOpts values before this patch.
[to be coupled with D61616]

Reviewers: chandlerc

Subscribers: jlebar, cfe-commits, llvm-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D61617

llvm-svn: 361534