RFR(M): 8238759: Clones should always keep the base pointer

[Roman Kennke]

Hi Nils,

This is a welcome change. We've been using the base in Shenandoah and
required to pull this out of the interior address. This change makes
this more robust.

The change looks good.

Thanks,
Roman

> This patch changes how clone uses arraycopy nodes. Normal arraycopy
> nodes have the base pointer as src. Arraycopys used for cloning have an
> interior pointer as src, pointing at where the copy should start. It's
> not ideal that the arraycopy node is used differently, and it introduces
> extra complexity when the base pointer is needed at a later stage.
> 
> This patch changes clones to have the base in the src field, and puts
> the offset to the where we want to start copy, in the pos field.
> 
> This patch also changes shenandoah code.
> 
> https://bugs.openjdk.java.net/browse/JDK-8238759
> http://cr.openjdk.java.net/~neliasso/8238759/webrev.01/
> 
> Please review,
> Nils Eliasson
>