Nullary comb.concat doesn't roundtrip

`"comb.concat"() : () -> (i0)` is valid generic syntax, but doesn't roundtrip through circt-opt - passing it through gives:

```
module {
  %0 = comb.concat  : 
}
```

on which circt-opt errors:

```
<stdin>:2:22: error: expected non-function type
  %0 = comb.concat  : 
                     ^
```

Easiest fix might be to add a concat verifier that rejects the nullary case?
