* xpath.c: Optimized xmlXPathNodeCollectAndTest() and
  xmlXPathNodeCollectAndTestNth() to evaluate a compound
  traversal of 2 axes when we have a "//foo" expression.
  This is done with a rewrite of the XPath AST in
  xmlXPathRewriteDOSExpression(); I added an additional field
  to xmlXPathStepOp for this (but the field's name should be
  changed). The mechanism: the embracing descendant-or-self
  axis traversal (also optimized to return only nodes which
  can hold elements), will produce context nodes for the
  inner traversal of the child axis. This way we avoid a full
  node-collecting traversal of the descendant-or-self axis.
  Some tests indicate that this can reduce execution time of
  "//foo" to 50%. Together with the XPath object cache this
  all significantly speeds up libxslt.