Reworks same-selector merging to run both ways.

Reworks same-selector merging to run both ways.

This way it can optimize more use cases as content may not be
movable one way but it could be other way, e.g.:

```css
a{color:red}
p{display:block}
a{display:inline-block}
```

In this case the third selector can't be moved and merged with the
first one, but it can be done the other way around.