C2: fix zeroing elimination with heap allocated value types

[Roland Westrelin]

Hi Tobias,

Thanks for looking at this.

> This looks good to me! Do we still need the call to
> set_complete_with_arraycopy() in ValueTypeNode::store_to_memory() that
> I tried to add in my value type array prototype [1]?

I don't know. I haven't checked yet.

Roland.