Hitting * after a visual selection is made doesn't highlight the matches

Hi,

Steps to reproduce with Vim 8.0:

1. Create a file with this content:

```
hello world
hello world
```

2. Visually select the first line's hello world

3. Press `*`

4. Nothing is highlighted but the command window will show `/\Vhello world`

I would have expected both hello world lines to be highlighted which is the behavior with `vim-visual-star-search` (which I uninstalled to avoid conflicts).

It's also worth pointing out if I move the cursor over the word hello and press `*` then it immediately highlights both hello strings, so this seems isolated to visually selecting words.

Also, if I press `n` after visually selecting text then the highlight happens but that seems like an unexpected event.

Am I doing something wrong? It's supposed to highlight the match after only pressing `*` right?