Done conditions are special and not quite like assignments. This PR updates the ir::Group structure to make done conditions a field so that we don't have to do the janky "iterate over the group assignments to find the done condition" nonsense.

Also, more importantly, we no longer allow a group's done condition to have guards. The textual programs can still write something like:

group upd {
  upd[done] = c.done & d.done ? 1
}

But the compiler immediately canonicalizes it to:

cells {
  dwc = wire(1)
}
group upd {
  upd[done] = dwc.out
}
dwc.in = c.done & d.done ? 1;

There are a couple of benefits of this change. First, we don't need the group[done] hole anymore; we can simply inline the group's done condition directly into the location since the condition is guaranteed to be a port. It also makes sure that figuring out if a group is done straightforward since it does not require any computation.

Finally, w.r.t. undefined behavior, it clarifies the responsibility of the port–it must always have a non-x value. The guard does not play a role in defining the "doneness" of a group.

Fixes #1119 and fixes #864.

TODO

• Fix rewriting of ports
• Update tests to have done conditions
• ReadWriteSet::uses on group assignments returns different results