JDK-8240696 added the native method for `Reference.clear`. The original patch skipped intrinsification of this method, because we thought `Reference.clear` is not on a performance sensitive path. However, it shows up prominently on simple locking benchmarks, especially when the `JVM_ReferenceClear` requires the WX transition, see attached profiler screenshot.

So, it would be good to intrinsify `Reference.clear` at least in C2.