[SLPVectorizer] Set flag to previous default.
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.
Subscribers: mehdi_amini, jlebar, eraman, steven_wu, dexonsmith, llvm-commits
Differential Revision: https://reviews.llvm.org/D61955