Function unpack_pair 

fn unpack_pair(src: u32x8) -> (u32x8, u32x8)

Unpack 32-bit lanes into 64-bit lanes:

(a0, b0, a1, b1, c0, d0, c1, d1)

into

(a0, 0, b0, 0, c0, 0, d0, 0)
(a1, 0, b1, 0, c1, 0, d1, 0)