More minmax vectorization

[StephanTLavavej]

Followup issue after merging #2447.

• Vectorize ranges::min_element, ranges::max_element, ranges::minmax_element
  • This should be similar to vectorizing ranges::count, as we should be able to call the same vector_algorithms.cpp functions after dealing with the iterator/sentinel. (Unlike ranges::find, we don't need to worry about unbounded ranges.)

And as @AlexGuteniev explained, we can go even further:

• Vectorize ranges::min, ranges::max, ranges::minmax
  • These algorithms return values instead of iterators, so they can be simpler and faster, and "will consist only of vertical max and one reduction".
• Use AVX2.

#2439 tracks extending the optimization to floating-point types.