The 8x4 and 4x4 use a needlessly large multiplier (unless/until we care
about embedded 64-bit-vector hardware). This is merely suboptimal.

The 8x4 case also uses an incorrect vector length, which leads to incorrect
behaviour on future/hypothetical hardware with 256-bit or larger vectors.
Pointed-out-by: Martin Storsjö <martin@martin.st>