a tiny bug in DehoistArrayIndex

R=mmassi@chromium.org

Review URL: https://codereview.chromium.org/15268009

Patch from Weiliang Lin <weiliang.lin2@gmail.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14733 ce2b1a6d-e550-0410-aec6-3dcde31c8c00