Compute 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 in result.
Compute 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 in result.