[IR] Dont push/pop dynscopes if not required.

* Added a REQUIRES_DYNSCOPE flag to record when a dynscope is
  required and use it to cleanup some hardcoded tests in IRScope.

* Used the flag to avoid pushing/popping dyn-scope on the stack.
  This only matters for the interpreter where we dont run with an
  explicit call protocol like in the JIT.

* Specialized InterpretedIRMethod for the case when it is used to
  interpret a meta class body since it has a special requirement
  for dyn-scopes.