Vertically multiply each unsigned 8-bit integer from a with the corresponding
signed 8-bit integer from b, producing intermediate signed 16-bit integers.
Horizontally add adjacent pairs of intermediate signed 16-bit integers,
and pack the saturated results.
Vertically multiply each unsigned 8-bit integer from a with the corresponding signed 8-bit integer from b, producing intermediate signed 16-bit integers. Horizontally add adjacent pairs of intermediate signed 16-bit integers, and pack the saturated results.