Faster AVX2 dot product for IQ2_XS (#5187)
* iq2xs: faster AVX2 dot product
* iq2xs: small AVX2 imrovement
* Speed up computing sign bits in AVX2 iq2_xs dot product
---------
Co-authored-by: Iwan Kawrakow <iwan.kawrakow@gmail.com>
Co-authored-by: Peter Reid <peter@peterreid.net>
