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

[Roman Kennke]

Looks good. Thanks!

Roman


> 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
>