[MachineOutliner] Never add the outliner in -O0

We shouldn't add the outliner when compiling at -O0 even if
-enable-machine-outliner is passed in. This makes sure that we
don't add it in this case.

This also updates machine-outliner-flags to reflect the change
and improves the comment describing what that test does.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@335879 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed