AArch64: gc/shenandoah/TestVerifyJCStress.java fails intermittently with C1

The #2 configuration (-XX:ShenandoahGCMode=iu) fails occasionally on some AArch64 machines when run with -XX:TieredStopAtLevel=1.

Looks like ShenandoahBarrierSetAssembler::storeval_barrier() needs to save more registers (all the call clobbered ones).

java.util.concurrent.ExecutionException: java.lang.NullPointerException: Cannot assign field "prev" because "node" is null
    at java.base/java.util.concurrent.FutureTask.report(FutureTask.java:122)
    at java.base/java.util.concurrent.FutureTask.get(FutureTask.java:191)
    at TestVerifyJCStress.main(TestVerifyJCStress.java:109)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:64)
    at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.base/java.lang.reflect.Method.invoke(Method.java:564)
    at com.sun.javatest.regtest.agent.MainWrapper$MainThread.run(MainWrapper.java:127)
    at java.base/java.lang.Thread.run(Thread.java:832)
Caused by: java.lang.NullPointerException: Cannot assign field "prev" because "node" is null
    at java.base/java.util.concurrent.locks.StampedLock.acquireWrite(StampedLock.java:1221)
    at java.base/java.util.concurrent.locks.StampedLock.writeLockInterruptibly(StampedLock.java:536)
    at TestVerifyJCStress$Test.WLI_Us(TestVerifyJCStress.java:133)
    at TestVerifyJCStress.lambda$main$2(TestVerifyJCStress.java:104)
    at java.base/java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:515)
    at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
    at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1130)
    at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:630)
    ... 1 moreJavaTest Message: Test threw exception: java.util.concurrent.ExecutionException: java.lang.NullPointerException: Cannot assign field "prev" because "node" is null