optimise $ref1 = $ref2 better

optimise $ref1 = $ref2 better

When assigning to a ref, the old referent is mortalised if its refcount
is 1, to avoid a premature free on things like $r = $$r or $r = $r->[0].

For the shortcut case where $ref1 and $ref2 are simple refs (no magic etc)
it's possible to do the assign then SvREFCNT_dec() the old value without
having to mortalise it. Which is faster.

Even when it doesn't have to be mortalised (RC > 1) this commit makes it
slightly faster as it no longer calls sv_unref_flags().

Conversely, this commit also makes the short-cut integer assign code path
infinitesimally slower.