Fix SML/NJ build for Backend

SML/NJ complained about an unresolved flex record in the mkArgOperands
function.  Its type inference didn't propigate what other fields would
be passed to #2 (in this case, callReturnOperands would pass it a
(Var.t * Type.t)).  Using pattern matching to grab the second element of
the tuple seemed the most straightforward, but specifying that 'args' is
a (Var.t * Type.t) vector would have also worked.