assign_op with overloaded operator evaluates receiver twice

[catamorphism]

The user-defined operator case in trans::base::trans_assign_op passes an expr evaluating to the callee's address, rather than the callee address, to trans_call_inner. Since the callee is known, this should be the statically known callee address (as per a FIXME).

[nikomatsakis]

This is much worse than this description makes it sound. If I'm reading the code correctly, what's actually happening here is that expr_x += expr_y results in expr_x being trans'd twice (e.g., foo().x += 3 would call foo() twice) if + is overloaded. The proper fix is to refactor the impl::trans_method_callee callee function to take a pre-evaluated self expression, I think, or perhaps have it be a shallow wrapper around a function which does.

[pcwalton]

I have a PR to fix this by removing overloaded operator assign ops altogether.

[thestinger]

These are removed for the time being.