Optimization to avoid a temporary for x <op>= y works for any RHS
which doesn't emit code when evaluated to an ivalue, e.g.:

* A plain constant or any expression which constant folds to a
  constant, e.g.: x += 4 and x += 'foo' + 'bar'.

* A register-bound variable, e.g. x += y.

The optimization doesn't have enough state to detect safe cases
such as register bound 'y' in: x += y + 1.

* A few RegExp issues have been resolved via ES6 RegExp syntax.