[driver] Disable the slp vectorizer at -O0, -O1, and -Oz.  This mirrors the
behavior of the loop vectorizer, which is enabled at -O2, -O3, -O4, -Ofast
and -Os.
PR19568

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@207433 91177308-0d34-0410-b5e6-96231b3b80d8