[MachineOutliner][NFC] Move decrement logic to OutlinedFunction

[MachineOutliner][NFC] Move decrement logic to OutlinedFunction

This commit moves the decrement logic for outlined functions into the class,
and makes OccurrenceCount private. It can now be accessed via
getOccurrenceCount().

This makes it more difficult to accidentally introduce bugs by incorrectly
decrementing the occurrence count on OutlinedFunctions.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@316020 91177308-0d34-0410-b5e6-96231b3b80d8