o The type recording binary stub got a new type for string + string.

o Added HStringAdd and LStringAdd based on the new type info.

o Started using HValue types to avoid unneccesary checks.

Review URL: http://codereview.chromium.org/6852015

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@7622 ce2b1a6d-e550-0410-aec6-3dcde31c8c00