Available on

**x86 and target feature**only.`sse2`

## Expand description

Sum the absolute differences of packed unsigned 8-bit integers.

Computes the absolute differences of packed unsigned 8-bit integers in `a`

and `b`

, then horizontally sum each consecutive 8 differences to produce
two unsigned 16-bit integers, and pack these unsigned 16-bit integers in
the low 16 bits of 64-bit elements returned.