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

[Nils Eliasson]

Hi,

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