When splitting at the beginning of a use interval assign coinciding position to the split child instead of leaving it to parent.

BUG=v8:982

Review URL: http://codereview.chromium.org/5898001

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@6031 ce2b1a6d-e550-0410-aec6-3dcde31c8c00