Avoid repeated rewrites of global store to constant IC due to store of same value.

R=mvstanton@chromium.org

Review URL: https://chromiumcodereview.appspot.com/19663007

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@15806 ce2b1a6d-e550-0410-aec6-3dcde31c8c00