Formerly we punted on these and evaluated constructors always got a tag
of 1.

We now cascade switches because we have to check the tag first and when
it is MAX_PTR_TAG then get the precise tag from the info table and
switch on that. The only technically tricky part is that the default
case needs (logical) duplication. To do this we emit an extra label for
it and branch to that from the second switch. This avoids duplicated
codegen.

Here's a simple example of the new code gen:

    data D = D1 | D2 | D3 | D4 | D5 | D6 | D7 | D8

On a 64-bit system previously all constructors would be tagged 1. With
the new code gen D7 and D8 are tagged 7:

    [Lib.D7_con_entry() {
         ...
         {offset
           c1eu: // global
               R1 = R1 + 7;
               call (P64[Sp])(R1) args: 8, res: 0, upd: 8;
         }
     }]

    [Lib.D8_con_entry() {
         ...
         {offset
           c1ez: // global
               R1 = R1 + 7;
               cal...