iq1bn: faster AVX2

Instead of shuffling quant data into a 128-bit register containing
8-bit ints, and then converting to 16 bit, we directly shuffle into
a 256-bit register containing 16 bit ints.

TG-128 @ 2 threads goes from 18.3 to 21.6 t/s.
TG-128 performance now saturates already at 8 threads getting 60.4 t/s.
There is almost no impact on PP-512 (322 -> 323 t/s). I guess,
we amortize dequantization cost pretty well, so we don't gain much
there.

We get close to 100 GB/s single-threaded float32 throuput:

./bin/test-quantize-perf --op vec_dot_q -i 10000000 --type iq1_bn
iq1_bn
  vec_dot_q
    4096 values (0.02 MB)
      min cycles/32 vals   :      3.87
      avg cycles/32 vals   :      4.40
      float32 throughput   :     98.27 GB/s
      quantized throughput :      4.99 GB/s

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