RFR (S) 8231244: Shenandoah: all-roots heap walking misses some weak roots

[Aleksey Shipilev]

Bug:
  https://bugs.openjdk.java.net/browse/JDK-8231244

Fix:
  https://cr.openjdk.java.net/~shade/8231244/webrev.02/

This reliably reproduces with vmTestbase_nsk_jvmti. We miss some weak roots, apparently. New code
tries to match how ShenandoahRoot{Evacuator,Adjuster}::roots_do handle it.

Testing: hotspot_gc_shenandoah, vmTestbase_nsk_jvmti

-- 
Thanks,
-Aleksey