Sink all of the code relying on the MachO MachineModuleInfo to live

behind the test that the MachineModuleInfo analysis was
actually available and can be used.

While the MachO bits may well be reasonable to assume in the darwin
assembly printer, the analysis isn't constructively guaranteed anywhere
I could find so it seems safest to avoid crashing here.

This issue was found with PVS-Studio. Pretty sure the Clang Static
Anaylzer flags similar issues but we've probably never pointed it at
this code effectively.

llvm-svn: 285972