Use existing flag to mark HChange instructions that deoptimize on undefined.

Also there is no need to store the source representation explicitly.
Review URL: http://codereview.chromium.org/8066007

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@9472 ce2b1a6d-e550-0410-aec6-3dcde31c8c00