Allow uncacheable identifiers to go generic.

BUG=v8:2867
R=jkummerow@chromium.org

Review URL: https://chromiumcodereview.appspot.com/23453019

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16481 ce2b1a6d-e550-0410-aec6-3dcde31c8c00