Optimize std::advance for single increments

	* include/bits/stl_iterator_base_funcs.h
	(__advance<_RandomAccessIterator, _Distance>): Optimize for next/prev
	cases where incrementing or decrementing a single step.

From-SVN: r248875

libstdc++-v3/ChangeLog

 2017-06-05  Jonathan Wakely  <jwakely@redhat.com>
 
+	* include/bits/stl_iterator_base_funcs.h
+	(__advance<_RandomAccessIterator, _Distance>): Optimize for next/prev
+	cases where incrementing or decrementing a single step.
+
 	* include/bits/shared_ptr_base.h (__shared_ptr::owner_before)
 	(__weak_ptr::owner_before, _Sp_owner_less::operator()): Add noexcept
 	specifiers as per LWG 2873 and LWG 2942.

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

@@ -177,7 +177,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       // concept requirements
       __glibcxx_function_requires(_RandomAccessIteratorConcept<
 				  _RandomAccessIterator>)
-      __i += __n;
+      if (__builtin_constant_p(__n) && __n == 1)
+	++__i;
+      else if (__builtin_constant_p(__n) && __n == -1)
+	--__i;
+      else
+	__i += __n;
     }
 
   /**