deminout: prevent any constant assignment from demoting to input

deminout: prevent any constant assignment from demoting to input

Before this patch,

```
module top(inout io);
assign io = 1'bx;
endmodule
```

would have the `io` pin demoted to input (same happens for `1'bz`,
but not for `1'b0` or `1'b1`), resulting in check failures later on.

Part of fix for #1841.