[vector] Update to Shuffle specification

[Paul Sandoz]

Hi,

See this patch for an update to the Shuffle specification (as previously discussed):

  http://cr.openjdk.java.net/~psandoz/panama/shuffle-shuffle/webrev/ <http://cr.openjdk.java.net/~psandoz/panama/shuffle-shuffle/webrev/>

I have not adjusted the internal represent to leverage ByteVector. That will require further investigation and may require an intrinsic to create a ByteVector from an int[] array (perhaps similar to creating a Mask from a boolean[]).

Paul.