Make THH:genSaveOpCode a bit more general

This can be improved, by taking all hardcoded names as parameters, to
serve as a more-general "build save clause for a multi-constructor
data type". I'm not renaming the function as well, since I don't know
exactly how much we can abstract later.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Helga Velroyen <helgav@google.com>