XPath: Extend the descendant-or-self optimization

Use descendant-or-self::node() transformation for self, descendant and
descendant-or-self axis. Self axis should be semi-frequent; descendant
axes should not really be used with // but if they ever are the complexity
of the step becomes quadratic so it's better to optimize this if possible.

git-svn-id: https://pugixml.googlecode.com/svn/trunk@1063 99668b35-9821-0410-8761-19e4c4f06640