[MachineOutliner][NFC] Don't add MBBs with a size < 2 to the search space

The suffix tree won't ever consider sequences with a length less than 2.

Therefore, we really ought to not even consider them in the first place.

Also add a FIXME explaining that this should be defined in terms of the size
in B of an outlined call versus the size in B of the MBB.

llvm-svn: 342688