Add two more cases of double arithmetic assembly instructions in which the
immediate argument is an expression rather than a simple integer.  It
appears the problem is not that dsubu fails to take an expression with
parenthesis (this works fine with daddiu) but rather that it is not being
converted to a daddiu by virtue of having an immediate rather than register
argument.  This does work for simple immediates with dsubu, just not full
expressions.