Fix bad comment from r207573.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@207854 91177308-0d34-0410-b5e6-96231b3b80d8
[driver] Enable the slp vectorizer at -Oz.
PR19568

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@207858 91177308-0d34-0410-b5e6-96231b3b80d8