`With_valid.t` gives names that are not supported by VHDL generation

Take for example a field:

```ocaml
module O = struct
  type 'a t =
    {
      num_zeros : 'a With_valid.t [@bits num_bits]
    }
  [@@deriving hardcaml]
end
```
On RTL generation, we get:

`"[Rtl_name.add_port_name] illegal port name" (name num_zeros$valid)`

This seems wrong, as the `With_valid.t` interface can't be used when generating VHDL.