Improve compilation of some multiple-assignment targets.

* All targets are non-expression, so remove variability there.
* AttrAsgn targets can propagate non-expression and be faster in
invokedynamic logic (no need to peel Nth argument off for return).

More work can be done to simplify all targets into non-expression
forms, which will then omit the popping of the last value and
possibly cascade non-expression behavior to contained elements.