Refactored out the helper method FindPredecessorAutoreleaseWithSafePath from ObjCARCOpt::OptimizeReturns.

Now ObjCARCOpt::OptimizeReturns is easy to read and reason about.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@178715 91177308-0d34-0410-b5e6-96231b3b80d8