Instance variable call sites currently attempt to cache in a way similar to method calls. However, they do not ever age out (i.e. give up trying to update the indy call site) and as a result sites that encounter a large number of types will either grow the PIC forever or continually blow out the call site cache.

This needs to be fixed for JRuby 1.7, so that megamorphic ivar call sites failover to a simple inline cache.

An example of invokedynamic.log.binding output showing the continual rebinding of ivars is here: http://pastie.org/3855673