[vector] Vector.cast

[John Rose]

Yes, that makes sense. A length changes should be a separate operation, which will generate very different code. 

> On May 25, 2018, at 3:08 PM, Paul Sandoz <paul.sandoz at oracle.com> wrote:
> 
> Hi,
> 
> Would it make sense to simplify Vector.cast to species of the same length? if length differs an exception is thrown.
> 
> That’s one less thing to think about and same concept can then be applied to Shuffle and Mask, which is effectively a nop if no boxing occurs because the underlying lane elements don't change.
> 
> We can lean on the reshape/rebracket/resize for other length transforming operations, in addition to mask/shuffle conversion to/from vector.
> 
> Paul.