Extend alias.c arrow type short-circuit to TK_BOX and TK_TAG #4969
Description
In #4963, alias() in alias.c was given a short-circuit for val->A arrow types — since a val viewpoint always produces val or tag (both self-aliasing), aliasing inside the arrow is equivalent to aliasing outside it, so the arrow is returned unchanged.
The same reasoning applies to box->A and tag->A arrow types. box->X always produces box, val, or tag (all self-aliasing). tag->X always produces tag (self-aliasing). So alias(box->A) and alias(tag->A) could also short-circuit.
No stdlib code currently exercises these paths, so this is a latent issue — it would only surface if someone writes code with box->A or tag->A types that go through aliasing and the over-conservative result causes a type error.