<rdar://problem/3997958> 
        REGRESSION (Mail): Mail takes half of forever to paste >1500 lines - replaceSelectionWithNode

        * dom/Position.cpp:
        (WebCore::Position::upstream): Avoid calling previous() when we know that 
        it will 1) end the search and 2) be expensive to compute.
        (WebCore::Position::downstream): Removed some dead code.
        (WebCore::Position::inRenderedText): Return false for offsets inside composed characters.
        * dom/Position.h:
        * editing/VisiblePosition.cpp:
        (WebCore::VisiblePosition::init): If there are two visually equivalent candidates, we choose
        the one that occurs first in document order.  Using upstream() to find the one that occurs first is
        much faster than the old code.



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@13412 268f45cc-cd09-0410-ab3c-d52691b4dbfc