[MachineOutliner] Add always and never options to -enable-machine-outliner

This is a recommit of r335887, which was erroneously committed earlier.

To enable the MachineOutliner by default on AArch64, we need to be able to
disable the MachineOutliner and also provide an option to "always" enable the
outliner.

This adds that capability. It allows the user to still use the old
-enable-machine-outliner option, which defaults to "always". This is building
up to allowing the user to specify "always" versus the target default
outlining behaviour.

https://reviews.llvm.org/D48682

llvm-svn: 335986