We get a slightly better PPL for LLaMA-3.1-8B compared to q6_K
(0.14% vs 0.26% quantization error).

90.2 t/s for LLaMA-3.1-8B. Q6_K gives 91.2 t/s, so we are good.

We need to do 4 shuffles to get the non-uniform values, so this
makes it slower than other iqX_k quants.

And then I realized that I was using the standard Zen4 template for
all iqX_k quants. The standard template converts the 32-bit integers
obtained after _mm512_dpbusds_epi32 back to 16 bits, and then multiples
with 16-bit block scales. But this can overfow for iq4_k, iq5_k, and
iq6_k. I guess, I did not notice with iq4_k and iq5_k because the
PPL difference to CUDA was relatively small, and I attributed it to
Q8_K not being accurate enough for the activations. But for iq6_k
the PPL difference was much too big to be attributable to Q8_K
inaccuracies, so that's when I realized that I cannot be packing
the _mm512_dpbusds_epi32 result into 16 bit for 4-,5-,6-bit iqX_k
quants.

For now I fixed it for iq6_k, but the outcome is that it is
significantly slower than Q6_K: I get PP-512 = 125 t/s for
LLaMA-3.1-8B vs 180 t/s for Q6_K, so I need to look for a better
approach.

We now arrive at pp-512 = 147 t/s for LLaMA-3.1-8B.
TG-128 is 9.5 t/s. This is better than last commit,
but still kind of slow compared to Q6_K.

My last commit message is wrong: also iq3_k needs a fix
for overflow.

Respectable performance, only slightly slower than Q6_K.

About 4% slower than Q6_K for PP-512, but 10% faster for TG-128.
Someone has screwed up Q6_K TG performance on Metal? With the
cobntinuous "improvements" in ggml I wouldn't be surprised.
Need to look into it later.

See comments in f3a823c