Reviewed by Kevin.

    Reviewed by Kevin.

	<rdar://problem/3807080> Safari so slow it seems like a hang accessing a page on an IBM website

        * khtml/xml/dom_nodeimpl.cpp:
        (NodeListImpl::NodeListImpl): Initialize isItemCacheValid, renamed isCacheValid to
	isLengthCacheValid.
        (NodeListImpl::recursiveLength): Adjusted for rename.
        (NodeListImpl::recursiveItem): Cache the last item accessed and its offset.
	If the same offset is looked up again, just return it, otherwise, if looking up
	a later offset, start at the last item and proceed from there.
        (NodeListImpl::itemById): Apply the special document optimization to all
	nodes that are either a document or in a document - just walk up to make
	sure the node found by ID has the root node as an ancestor.
        (NodeListImpl::rootNodeSubtreeModified): Adjust both cache bits.
        * khtml/xml/dom_nodeimpl.h: Prototype new stuff.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@8007 268f45cc-cd09-0410-ab3c-d52691b4dbfc