Species-agnostic shuffle

[Martin Traverso]

Hi,

I've been experimenting with the Vector API in an attempt to port the XXH3
hashing algorithm to Java. One of the steps requires rearranging the
elements of a vector by swapping adjacent values. I.e., (v0, v1, v2, v3,
...) -> (v1, v0, v3, v2, ...).

One usability issue I'm running into is creating the VectorShuffle in a
species-agnostic manner. The VectorShuffle.fromArray() expects a number of
indices that matches *exactly* the length of the vector for the given
species, so it's harder to create a shuffle that uses SPECIES_PREFERRED
without additional logic to construct that array of indices dynamically.

To illustrate, I need to do the following for each species:

    VectorShuffle.fromArray(LongVector.SPECIES_128, new int[] { 1, 0 }, 0)
    VectorShuffle.fromArray(LongVector.SPECIES_256, new int[] { 1, 0, 3, 2
}, 0)
    VectorShuffle.fromArray(LongVector.SPECIES_512, new int[] { 1, 0, 3, 2,
5, 4, 7, 6 }, 0)

Of course, I could make something like this

    VectorShuffle.fromArray(LongVector.SPECIES_PREFERRED,
makeIndexesFromTemplate(new int[] { 1, 0 }), 0)

with my own custom "makeIndexesFromTemplate" function to create the array
of indices of the appropriate length.

Is there anything like that in the Vector APIs that I haven't been able to
find? If not, would it make sense to add something like it, or allow
VectorShuffle.fromXXX to derive such mapping?

- Martin
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