While investigating GC time in libgraal for InvokeVirtualICCE.java in Xcomp mode I discovered that we were accumulating WeakReferences that had been cleared. Because that test is loading and unloading classes continuously it can accumulate a significant number and since it's running in Xcomp that isolate never goes idle. In a normal Xmixed execution the we would still accumulate them but not as many and the command shutdown of idle isolates would through away that heap.

The fix is to use a ReferenceQueue to track their release and clean the map values when some have been reclaimed.