Lazily evaluate the right-hand side in augmented assignment

[d-torrance]

This is a quick followup to #3079. We hold off on evaluating y in something like x += y until we absolutely know we need it. With this change, ??= would work out of the box if we were to add ?? (or whatever) as a null coalescing operator -- see #1596.

[d-torrance]

Wait, what I said above isn't quite correct -- ??= wouldn't work out of the box for x ??= y if evaluating x raises an error (e.g., a nonexistent key in a hash table)., but it would work out of the box if x is null. It should be an easy change to make it work for errors, though.