libstc++: Implement hmin and hmax

libstc++: Implement hmin and hmax

From 9.7.4 in Parallelism TS 2. For some reason I overlooked these two
functions. Implement them via call to _S_reduce.

libstdc++-v3/ChangeLog:

	* include/experimental/bits/simd.h: Add __detail::_Minimum and
	__detail::_Maximum to use them as _BinaryOperation to _S_reduce.
	Add hmin and hmax overloads for simd and const_where_expression.
	* include/experimental/bits/simd_scalar.h
	(_SimdImplScalar::_S_reduce): Make unused _BinaryOperation
	parameter const-ref to allow calling _S_reduce with an rvalue.
	* testsuite/experimental/simd/tests/reductions.cc: Add tests for
	hmin and hmax. Since the compiler statically determined that all
	tests pass, repeat the test after a call to make_value_unknown.