optimize() causes segment cache to be wrong when merging <A>

[JiuqingSong]

If a link contains multiple segments, for example

<a>
  <img>
   text
</a>

When first write DOM, we will create two <A> nodes then merge them. However, when merge, their indexes are not merged.

Actually I don't think we really need to save index on <A>, but only put them on leaf nodes should be enough