[MachineOutliner][NFC] Simplify and unify pruning/outlining logic

Since we're now performing outlining per OutlinedFunction rather than per
Candidate, we can simply outline each candidate as it shows up.

Instead of having a pruning phase, instead, we'll outline entire functions.
Then we'll update the UnsignedVec we mapped to reflect the deletion. If any
candidate is in a space that's marked dirty, then we'll drop it.

This lets us remove the pruning logic entirely, and greatly simplifies the
code.

llvm-svn: 348420