[MachineOutliner] Never add the outliner in -O0

This is a recommit of r335879.

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 removes -O0 from the outliner DWARF test.

llvm-svn: 335930