`minmax` 8 and 16 bit elements are not vectorized

[AlexGuteniev]

Oversight in initial implementation in #4384. Noticed while benchmarking #4659

Might be superseded by #4453.

The optimization is performed in _Minmax_fwd_unchecked. We don't pick this branch for smaller elements, because _Prefer_iterator_copies evaluates to false.

STL/stl/inc/algorithm

Lines 10417 to 10420 in 8dc4faa

	if constexpr (forward_range<_Rng> && _Prefer_iterator_copies<iterator_t<_Rng>>) {
	return _Minmax_fwd_unchecked(
	_STD move(_UFirst), _STD move(_ULast), _STD _Pass_fn(_Pred), _STD _Pass_fn(_Proj));
	} else {

[StephanTLavavej]

Great catch! We should be careful to avoid regressing perf for non-vectorized scenarios here.