This appears to not be a speed-up, but it seems like a good idea, so I want a bug for the idea. I will upload my patch.
Created attachment 223205 [details]
work in progress
This used to be a slow-down but now with all of the work on improving support for polymorphic heap accesses, this is now perf-neutral.
Created attachment 233722 [details]
Comment on attachment 233722 [details]
Landed in http://trac.webkit.org/changeset/170382