This speeds up constructs like this: 
var zz='replace'; '123'[zz]('3','4');

Review URL: http://codereview.chromium.org/2280007

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@4804 ce2b1a6d-e550-0410-aec6-3dcde31c8c00