After internal investigation, it’s no longer clear that the alternative
LowLevelHash mixer committed in a05366d8
unequivocally improves performance on AArch64. It unnecessarily reduces
performance on Apple Silicon and the AWS Graviton. It also lowers hash
quality, which offsets much of the performance gain it provides on the
Arm Neoverse N1 (see https://github.com/abseil/abseil-cpp/issues/1093).
Switch back to the original mixer.

Closes: https://github.com/abseil/abseil-cpp/issues/1093
PiperOrigin-RevId: 493941913
Change-Id: I84c789b2f88c91dec22f6f0f6e8c5129d2939a6f