Function cross_block_alignr_256x1 

unsafe fn cross_block_alignr_256x1(
    a: __m256i,
    b: __m256i,
    shift_bytes: usize,
) -> __m256i

Concatenates b and a (each 1 x __m256i = 2 blocks) and extracts 2 blocks starting at byte offset shift_bytes. Extracts from [b : a] (b in low bytes, a in high bytes), matching alignr semantics.