[MachineOutliner][NFC] Change getMachineOutlinerMBBFlags to isMBBSafeToOutlineFrom

Instead of returning Flags, return true if the MBB is safe to outline from.

This lets us check for unsafe situations, like say, in AArch64, X17 is live
across a MBB without being defined in that MBB. In that case, there's no point
in performing an instruction mapping.

llvm-svn: 346718