[MachineOutliner][NFC] Refactoring + comments in runOnModule

Split up some of the if/else branches in runOnModule. Elaborate on some
comments. Replace a call to getOrCreateMachineFunction with getMachineFunction.

This makes it clearer what's happening in runOnModule, and ensures that the
outliner doesn't create any MachineFunctions which will never be used by the
outliner (or anything else, really).

llvm-svn: 328240