[patch] Two fixes to the vpermilvar optimization

[Rafael Espíndola]

When I added instcomine logic to handle vpermilvar's pd and 256
variants I incorrectly assumed that the intel architecture was more
regular than what it actually is.

A coworker pointed out that the _256 variants use indexes into the
individual 128 bit lanes and while fixing that I noticed I also had to
mask out unused bits.

The attached patch fixes both issues.

Cheers,
Rafael
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