[MachineOutliner][NFC] Simplify logic in pruneCandidates

This commit yanks out the repeated sections of code in pruneCandidates into
two lambdas: ShouldSkipCandidate and Prune. This simplifies the logic in
pruneCandidates significantly, and reduces the chance of introducing bugs by
folding all of the shared logic into one place.

llvm-svn: 314475