libstdc++: Minor optimization for min/max/minmax

libstdc++: Minor optimization for min/max/minmax

The debug mode checks for a valid range are redundant when we have an
initializer_list argument, because we know it's a valid range already.
By making std::min(initialier_list<T>) call the internal __min_element
function directly we avoid a function call and skip those checks. The
same can be done for the overload taking a comparison function, and also
for the std::max and std::minmax overloads for initializer_list
arguments.

Signed-off-by: Jonathan Wakely <jwakely@redhat.com>
libstdc++-v3/ChangeLog:

	* include/bits/stl_algo.h (min(initializer_list<T>))
	(min(initializer_list<T>, Compare)): Call __min_element directly to
	avoid redundant debug checks for valid ranges.
	(max(initializer_list<T>), max(initializer_list<T>, Compare)):
	Likewise, for __max_element.
	(minmax(initializer_list<T>), minmax(initializer_list<T>, Compare)):
	Likewise, for __minmax_element.