patch: make instcombine remove shuffles by reordering vector elements

[Anton Korobeynikov]

>> It seems like it should be possible. If we restrict the problem so that
>> multiple simultaneous intermediate results are not allowed, then this
>> certainly seems like a group-theory question. Do you know anyone who is a
>> computational group theory expert?
> I still have an office in the Orsay mathematics department, a leftover from
> a
> previous life!  I can ask some of the people there next time I drop by.
Looks like a neat project!

--
With best regards, Anton Korobeynikov
Faculty of Mathematics and Mechanics, Saint Petersburg State University