Revert "If -fslp-vectorize or -fno-slp-vectorize are given, honor this...

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.

llvm-svn: 187633