Revert "If -fslp-vectorize or -fno-slp-vectorize are given, honor this selection. If no flag is given, enable it for -O3."

This reverts commit r187630.

Looks like it is breaking the bots.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@187633 91177308-0d34-0410-b5e6-96231b3b80d8