* Faster iq4_xs_r4 on Zen4
The trick is to simply prepare the Q8 block sums for
blocks of 32 as floats. This brings PP-512 up to 254.6 t/s
from 224 t/s.
* Fix broken matrix x vector product on Zen4
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Co-authored-by: Iwan Kawrakow <iwan.kawrakow@gmail.com>