util: throw if unreachable code is reached

If a comparison code path that is supposed to be unreachable is reached,
throw. Add a c8 comment to ignore coverage for the line, as it should be
unreachable.

PR-URL: https://github.com/nodejs/node/pull/31712


Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Anto Aravinth <anto.aravinth.cse@gmail.com>
Reviewed-By: Yosuke Furukawa <yosuke.furukawa@gmail.com>
Reviewed-By: Yongsheng Zhang <zyszys98@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>