Revert "[MachineOutliner] Never add the outliner in -O0"

This reverts commit 9c7c10e4073a0bc6a759ce5cd33afbac74930091.

It relies on r335872 since that introduces the machine outliner
flags test. I meant to commit D48683 in that commit, but got mixed
up and committed D48682 instead. So, I'm reverting this and
r335872, since D48682 hasn't made it through review yet.

llvm-svn: 335882