Shenandoah: assert(klass->is_loader_alive()) failed: must be alive

[Zhengyu Gu]

We have seen above assertion failures in shenandoah/jdk nightly tests 
periodically, especially after JDK-8230401.

It actually became quite reproducible on my local machine with 
tier3_gc_shenandoah after JDK-8230401.

The problem appears to be missing proper synchronization of metadata of 
newly registered nmethod.  Initially, I thought new nmethod is 
registered under CodeCahe_lock and concurrent codecache iteration always 
starts with acquiring CodeCache_lock lock. What I missed is that, the 
actual codecache iteration is done by worker thread, while the 
CodeCache_lock is acquired by VMThread. Therefore, it never establishes 
synchronization of newly created nmethod metadata (ShenandoahNMethod) 
between compiler thread (registering the nmethod) and worker thread 
(processing the nmethod).

The solution is to always acquire per-nmethod lock when changing and 
iterating the metadata.

Webrev: 
http://cr.openjdk.java.net/~zgu/shenandoah/assert_is_loader_alive/webrev.00/

Test:
   hotspot_gc_shenandoah (fastdebug and release)

   hotspot_gc_shenandoah/tier3_gc_shenandoah (fastdebug) with 10+ 
iterations on my local machine and gotland respectively.

Thanks,

-Zhengyu