Originally reported as DevCom-1560717

_Countr_zero in this loop has unnecessary checks for zero input in the compiled code:

See the demo with __assume workaround: https://godbolt.org/z/Ts7ExjbYb

As the reported noted, compiler codegen could be improved.

But from the library, it can also be fixed by making specialized _Countr_zero that does not check for zero input, and calling the specialized version from std::gcd.