Only allow changing hole to nan if all uses allow undefined as nan.

R=danno@chromium.org

Review URL: https://chromiumcodereview.appspot.com/22152003

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@16183 ce2b1a6d-e550-0410-aec6-3dcde31c8c00