Decode both planes of a block with VBMI byte permutes: the *3 multiply
chain (wrapping, so it commutes with the permutation) is computed once
on the whole 56-byte block, and masked vpermb picks each element's byte
from the chain vector of its base-3 digit. The qh lanes never see 3^4,
which would read the padding 5th digit of the qh bytes. The trits reach
the integer product as xi in {0, 1, 2} via the same avg trick as
tq1_0, with VNNI dpbusd against the q8_0 bytes and sum(y) subtracted.
The per-q8_0-block sums and the float accumulation keep the exact
operation order of the generic implementation, so the result is
bit-identical to it (checked by test-dt3).
2.2x over the (autovectorized) generic on a Ryzen AI 9 HX 370.
Millaguie
b110945afc