What's happening here then? Are these not two 128-bit integers? One's a 64-bit recasted to 128-bit, the other a 128-bit constant. Code would be doing faulty math, if it just decides to drop any bits. Coincidence, maybe, that the upper half of the recasted is in this case 0x0, but the code must work for 0xFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF as well, and probably does too.
tmp = (__uint128_t) wyhash64_x * 0xa3b195354a39b70d;