RFR(M): 8210887: Tweak C2 gc api for arraycopy

[Roland Westrelin]

Hi Tobias,

> Looks good to me. I'm assuming the currently unused arguments of array_copy_requires_gc_barriers
> will be used for Shenandoah? Why you need to distinguish between ArrayCopyPhase?

Thanks for the view.

Yes extra arguments are to be used by shenandoah.

Generating barriers once parsing is over is not supported by all
gcs. The shape of the barriers is sometimes too complicated to be
emitted at igvn time.

Roland.