Another CPS shrink

[Matthew Fluet]

This one I'm less sure about.  In 14.sml from the regression directory,
why can't we eliminate the constant equality test?  The CPS has

   val global_13 = A_0 ()
   val global_14 = B_0 ()

   fun L_42 () =
      let
         val x_7 = Env_0 ()
         val x_6 = Ref_assign (x_1, x_7)
         val x_5 = MLton_eq (global_13, global_14)
      in
         case x_5 of
           false => L_32 | true => L_31
      end