Function repack_pair 

fn repack_pair(x: u32x8, y: u32x8) -> u32x8

Repack 64-bit lanes into 32-bit lanes:

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

into

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