xxh64_merge_round
h64 = xxh64_merge_round(h64, v1);
h64 = xxh64_merge_round(h64, v2);
h64 = xxh64_merge_round(h64, v3);
h64 = xxh64_merge_round(h64, v4);
h64 = xxh64_merge_round(h64, v1);
h64 = xxh64_merge_round(h64, v2);
h64 = xxh64_merge_round(h64, v3);
h64 = xxh64_merge_round(h64, v4);