Use temporary variable for generic case instead for better IL

Author: alexcovington

src/libraries/System.Private.CoreLib/src/System/SpanHelpers.cs

@@ -594,7 +594,9 @@ private static void ReverseInner<T>(ref T elements, nuint length)
             ref T last = ref Unsafe.Subtract(ref Unsafe.Add(ref first, (int)length), 1);
             do
             {
-                (last, first) = (first, last);
+                T temp = first;
+                first = last;
+                last = temp;
                 first = ref Unsafe.Add(ref first, 1);
                 last = ref Unsafe.Subtract(ref last, 1);
             } while (Unsafe.IsAddressLessThan(ref first, ref last));