`<numeric>`: `_Countr_zero` used by `std::gcd` has unnecessary zero checks

[AlexGuteniev]

Originally reported as DevCom-1560717

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

STL/stl/inc/numeric

Lines 567 to 577 in d8f03cf

	_Mx_magnitude >>= _Mx_trailing_zeroes;
	do {
	_Nx_magnitude >>= static_cast<unsigned long>(_Countr_zero(_Nx_magnitude));
	if (_Mx_magnitude > _Nx_magnitude) {
	_Common_unsigned _Temp = _Mx_magnitude;
	_Mx_magnitude = _Nx_magnitude;
	_Nx_magnitude = _Temp;
	}
	_Nx_magnitude -= _Mx_magnitude;
	} while (_Nx_magnitude != 0U);

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.