Switch code flushing to use different JSFunction field.

Switch code flushing to use different JSFunction field.

This is another preparation for incremental code flushing. Instead of
linking candidates using the code entry field, we use the next pointer
that is also used to link optimized functions together. Since we only
support flushing of unoptimized code, this field can be shared.

R=ulan@chromium.org
BUG=v8:1609

Review URL: https://codereview.chromium.org/11141023

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@12734 ce2b1a6d-e550-0410-aec6-3dcde31c8c00