[objc-arc] Refactored OptimizeReturns so that it uses continue instead of a large multi-level nested if statement.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@179964 91177308-0d34-0410-b5e6-96231b3b80d8
[objc-arc] Fixed comment typo.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@179965 91177308-0d34-0410-b5e6-96231b3b80d8