Tag stubs that rely on instance types as MEGAMORPHIC.

BUG=chromium:173974

Review URL: https://chromiumcodereview.appspot.com/12178017

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13586 ce2b1a6d-e550-0410-aec6-3dcde31c8c00