Enable optimization of arraycopy as loads/stores with Shenandoah

[Roland Westrelin]

> Now that you say it, I wonder what's done for other GCs. They must be
> doing something here, to update the card tables. Other arraycopy
> routines call a special barrier in
> BarrierSet::static_write_ref_array_post(), this is not suitable for
> clones, but do they call any barrier for clone too? Or can other GCs
> ignore it because it's basically initializing stores?

For clone, unless ReduceInitialCardMarks is false, nothing is done AFAICT.

Roland.