RFR: 8242130: Shenandoah: Simplify arraycopy-barrier dispatching

[Roman Kennke]

>> Issue:
>> https://bugs.openjdk.java.net/browse/JDK-8242130
>> Webrev:
>> http://cr.openjdk.java.net/~rkennke/JDK-8242130/webrev.00/
> 
> *) Is it  actually safe to ignore dest_uninitialized path? Would the new arraycopy_marking routine
> walk over uninitialized memory now?

This is not about safety, it is about optimization: when the destination
array is uninitialized, we don't need to mark the previous values,
because they are known to be NULL.

Also, when dest is uninitialized, then we don't include MARKING in the
gc-state check above. Which means it would never trigger during marking,
and only during evac, in which case we're looking at the src array only.

Does that make sense?

Roman