Function cross_block_slide_blocks_at 

pub(crate) fn cross_block_slide_blocks_at<const N: usize, Block: Copy>(
    a: &[Block; N],
    b: &[Block; N],
    out_idx: usize,
    shift_bytes: usize,
) -> [Block; 2]

Selects the input operands to be used for slignr/vext/etc. when computing a single output block for cross-block “slide” operations. Extracts from [a : b].