Zap holes in dependent code array after deoptimizing a code group.

BUG=crash on GC stress builder

R=mstarzinger@chromium.org

Review URL: https://chromiumcodereview.appspot.com/12315077

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13715 ce2b1a6d-e550-0410-aec6-3dcde31c8c00