[vector] shuffle constructors

[Paul Sandoz]

Hi,

Please see this patch adding two shuffle constructors, one to create a shuffle using a lane index mapping function, and iota (and derived from the former using the identity function).

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

If such shuffles are used as constant values then i think we can provide the lambda accepting shuffle constructor to concisely express a rich set of rearrangements (reverse, rotate, shift, interleave, subset-reverse for byte swapping).

Thanks,
Paul.