XPath: Fix optimization bug with //name[last()]

The actual condition for the optimization is invariance from context list
-- this includes both position() and last().

Instead of splitting the posinv concept just include last() into
non-posinv expressions - this requires sorting for boolean predicates that
depend on last() and do not depend on position(). These cases should be
very rare.

git-svn-id: https://pugixml.googlecode.com/svn/trunk@1060 99668b35-9821-0410-8761-19e4c4f06640