Make sure to always fetch the RHS of a list assignment first, instead
of special casing known self-assignments, which will not detect cases
using references correctly.
As a side-effect, it is no longer possible to do something like
byRef(list($x) = $y). This worked by accident previously, but only
if $y was a CV and the self-assignment case did not trigger.
However it shouldn't work for the same reason that byRef($x = $y)
doesn't. Conversely byRef(list(&$x) = $y) and byRef($x =& $y)
continue to be legal.