Function cross_block_alignr_one 

unsafe fn cross_block_alignr_one(
    regs: &[__m256i],
    block_idx: usize,
    shift_bytes: usize,
) -> __m256i

Computes one output __m256i for cross_block_alignr_* operations.

Given an array of registers, each containing two 128-bit blocks, extracts two adjacent blocks (lo_idx and hi_idx = lo_idx + 1) and performs alignr with intra_shift.