The wwstage compound-deref arm narrowed the store for scalar pointees and
otherwise emitted a single MOVQ, so `*p OP= v` with p:*tagged clobbered
one word (the tag) and returned -- silently miscompiling what cstage
already rejects. A compound op on a whole union is nonsense. Gate the arm
on a scalar pointee size and let a tagged pointee fall through to the
existing assign-resolver reject, the byte-id twin of the cstage fatal.
cstage is unchanged.
This closes the deref member of the compound-on-tagged class; the index
and ident members (gs[i] OP= v, g OP= v) reject in a follow-up (#20/#21).
Hojun-Cho
769be55905