Shenandoah: assert(dmw.is_neutral()) failed: invariant: header=0xef000000279f2327

Summary is provisional, please update as appropriate.

Observed while running hotspot_gc_shenandoah (TestEvilSyncBug.java) on my machine:

# Internal Error (/home/rkennke/src/openjdk/jdk-jdk/src/hotspot/share/runtime/synchronizer.cpp:1577), pid=111737, tid=111748
# assert(dmw.is_neutral()) failed: invariant: header=0xef000000279f2327

See attached hs_err file. It happens during concurrent-weak-roots.