SIMD shuffle

# `simd.shuffle` rearranges lanes (owner, 2026-09-27: a): the vector, then its new order as
# lane numbers, which must be fixed numbers known when compiling. Reversing, swapping and
# copying one lane everywhere are all just orders.
MAIN {
    let 'xs': array[int32] = [10, 20, 30, 40];
    let 'v': simd[int32, 4] = simd.load[lend 'xs', pos 1];
    let 'r' := simd.shuffle['v', [4, 3, 2, 1]];         # reversed
    let 'pairs' := simd.shuffle['v', [2, 1, 4, 3]];     # neighbours swapped
    let 'first' := simd.shuffle['v', [1, 1, 1, 1]];     # lane 1 everywhere
    std::print.useful['r', newln, 'pairs', newln, 'first', newln];
}