Fix #2880 optimize() causes segment cache to be wrong when merging <A>

[JiuqingSong]

The problem is, when create DOM tree, content model index is added to not only leaf nodes (#text, image, br, ...) but also their wrapper if any (e.g. <A>). And optimize() can merge some elements without updating their index info. When reconcile content model from user's input, we may see an incorrect index to be updated.

To repro:

1. use the snapshot below
2. type something
3. press enter

After enter, the text order is wrong. This is because the cached model is incorrectly updated.

<a href="#"><img src="null">test</a><!--{"type":"range","start":[1],"end":[1],"isReverted":false,"isDarkMode":false}-->

To fix it:

1. No need to add model index to wrapper element (A, CODE, ...)
2. When reconcile, find index from leaf node only.