Merged r17772 into 3.22 branch.

Fix missing type feedback check for Generic*String addition.

BUG=318671
R=svenpanne@chromium.org
LOG=y

Review URL: https://codereview.chromium.org/73903002

git-svn-id: https://v8.googlecode.com/svn/branches/3.22@17779 ce2b1a6d-e550-0410-aec6-3dcde31c8c00