[SLPVectorizer] Set flag to previous default.

Summary:
The refactoring in r360276 moved the `RunSLPVectorization` flag and added the default explicitly. The default should have been `false`, as before.

The new pass manager used to have SLPVectorization on by default, now it's off in opt, and needs D61617 checked in to enable it in clang.

Reviewers: chandlerc

Subscribers: mehdi_amini, jlebar, eraman, steven_wu, dexonsmith, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D61955

llvm-svn: 361537