Refactor a generator out from part of SQLCompiler.get_order_by()

[Chris Jerdonek]

I noticed that SQLCompiler.get_order_by() has a 70-line ​chunk of code whose job is to build up an order_by list, which is then iterated over ​right after. That chunk of code has 8 order_by.append() lines and an ​order_by.extend(). It looks like this could be made cleaner by pulling those lines out into a private method that is instead a generator that yields those pairs. That would eliminate the need for having order_by.append() in several spots, and it would reduce the size of the method into two smaller methods.