Avoid visiting the subexpressions of a variable that rewrites to a property
when occurring as the 'left-hand side' of for/in.

BUG=v8:1149

Review URL: http://codereview.chromium.org/6475009

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@6772 ce2b1a6d-e550-0410-aec6-3dcde31c8c00