Before, expressions didn't take advantage of knowing their context in
the AST.  Now, we use the context to decide what to do with a value at
the end of visiting an expression.

Review URL: http://codereview.chromium.org/5620007

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@5954 ce2b1a6d-e550-0410-aec6-3dcde31c8c00