`undefined` as an unused value could be stripped

For example:

```js
x ? undefined : foo();
```

Start by reproducing the JavaScript example `x ? undefined : foo();` in the minifier and trace how conditional expressions with an unused value are handled. Add coverage showing that the unnecessary `undefined` value is stripped while the `foo()` branch remains correct; done means the transformed output preserves behavior and the regression test passes.

Written by the indexing model from the issue text.