Debug Info: code clean up.

Use EmitLabelOffsetDifference for handling on darwin platform when
non-darwin platforms use EmitLabelPlusOffset.

Also fix a bug in EmitLabelOffsetDifference where the size is hard-coded
to 4 even though Size is passed in as an argument.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@193660 91177308-0d34-0410-b5e6-96231b3b80d8