libstdc++: Micro-optimisations for lexicographical_compare_three_way

As noted in LWG 3410 the specification in the C++20 draft performs more
iterator comparisons than necessary when the end of either range is
reached. Our implementation followed that specification. This removes
the redundant comparisons so that we do no unnecessary work as soon as
we find that we've reached the end of either range.

The odd-looking return statement is because it generates better code
than the original version that copied the global constants.

	* include/bits/stl_algobase.h (lexicographical_compare_three_way):
	Avoid redundant iterator comparisons (LWG 3410).

libstdc++-v3/ChangeLog

+2020-03-03  Jonathan Wakely  <jwakely@redhat.com>
+
+	* include/bits/stl_algobase.h (lexicographical_compare_three_way):
+	Avoid redundant iterator comparisons (LWG 3410).
+
 2020-03-02  Jonathan Wakely  <jwakely@redhat.com>
 
 	PR libstdc++/93972

libstdc++-v3/include/bits/stl_algobase.h

@@ -1711,15 +1711,16 @@ _GLIBCXX_BEGIN_NAMESPACE_ALGO
 		return __lencmp;
 	      }
 #endif // is_constant_evaluated
-      while (__first1 != __last1 && __first2 != __last2)
+      while (__first1 != __last1)
 	{
+	  if (__first2 == __last2)
+	    return strong_ordering::greater;
 	  if (auto __cmp = __comp(*__first1, *__first2); __cmp != 0)
 	    return __cmp;
 	  ++__first1;
 	  ++__first2;
 	}
-      return __first1 != __last1 ? strong_ordering::greater
-	: __first2 != __last2 ? strong_ordering::less : strong_ordering::equal;
+      return (__first2 == __last2) <=> true; // See PR 94006
     }
 
   template<typename _InputIter1, typename _InputIter2>