We accidentally compared a map address with an instance type.  This
fix additionally avoids an upper bounds check that is not needed.

Review URL: http://codereview.chromium.org/149003

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@2272 ce2b1a6d-e550-0410-aec6-3dcde31c8c00