libstdc++: Extend memcmp optimization in std::lexicographical_compare

libstdc++: Extend memcmp optimization in std::lexicographical_compare

Make the memcmp optimization work for std::deque iterators and safe
iterators.

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

2020-06-08  François Dumont  <fdumont@gcc.gnu.org>
	    Jonathan Wakely  <jwakely@redhat.com>

	* include/bits/deque.tcc (__lex_cmp_dit): New.
	(__lexicographical_compare_aux1): Define overloads for deque
	iterators.
	* include/bits/stl_algobase.h (__lexicographical_compare::__3way):
	New static member function.
	(__lexicographical_compare<true>::__3way): Likewise.
	(__lexicographical_compare<true>::__lc): Use __3way.
	(__lexicographical_compare_aux): Rename to
	__lexicographical_compare_aux1 and declare overloads for deque
	iterators.
	(__lexicographical_compare_aux): Define new forwarding function
	that calls __lexicographical_compare_aux1 and declare new overloads
	for safe iterators.
	(lexicographical_compare): Do not use __niter_base on
	parameters.
	* include/debug/safe_iterator.tcc
	(__lexicographical_compare_aux): Define overloads for safe
	iterators.
	* testsuite/25_algorithms/lexicographical_compare/1.cc: Add
	checks with random access iterators.
	* testsuite/25_algorithms/lexicographical_compare/deque_iterators/1.cc:
	New test.