css: Don't reorder some selectors when building selector tree

css: Don't reorder some selectors when building selector tree

When building the tree we generally reorder the selectors inside
the same simple selector in order to pick a good first selector
to balance the tree better. However, some kinds of selectors
can't really be reordered, even thought they are simple.

This is since the matching code for some types handle
the existance of a directly preceeding selector differently:

 REGION and ANY selectors look for a DESCENDANT previous
 POSITION selector look for a REGION previous