[mcs] eval.cs: re-enable EvaluationListener codegen on OptionalAssign.EmitStatement

This important code was commented out to allegedly fix a bug
with compound statements (e.g. x++). I think the actual bug
was calling source.Emit instead of target.Emit which caused
duplicate code generation.

Regardless of whether or not source.Emit or target.Emit is
correct, the code generation for invoking the EvaluationListener
is very much necessary.