[MachineOutliner][NFC] Sink some candidate logic into OutlinedFunction

Just some simple gardening to improve clarity.

Before, we had something along the lines of

1) Create a std::vector of Candidates
2) Create an OutlinedFunction
3) Create a std::vector of pointers to Candidates
4) Copy those over to the OutlinedFunction and the Candidate list

Now, OutlinedFunctions create the Candidate pointers. They're still copied
over to the main list of Candidates, but it makes it a bit clearer what's
going on.

llvm-svn: 337838