Reuse cached task for -1.

Ix.NET/Source/System.Linq.Async/System/Linq/Operators/Reverse.cs

@@ -69,7 +69,7 @@ namespace System.Linq
 
                     if (!(_source is ICollection<TSource>) && !(_source is ICollection))
                     {
-                        return Task.FromResult(-1);
+                        return TaskExt.MinusOne;
                     }
                 }
 

Ix.NET/Source/System.Linq.Async/System/Linq/Operators/Select.cs

@@ -238,7 +238,7 @@ namespace System.Linq
             {
                 if (onlyIfCheap)
                 {
-                    return Task.FromResult(-1);
+                    return TaskExt.MinusOne;
                 }
 
                 var count = 0;

Ix.NET/Source/System.Linq.Async/System/Linq/Operators/Union.cs

@@ -182,7 +182,7 @@ namespace System.Linq
 
             public Task<List<TSource>> ToListAsync(CancellationToken cancellationToken) => FillSetAsync(cancellationToken).ContinueWith(set => set.Result.ToList());
 
-            public Task<int> GetCountAsync(bool onlyIfCheap, CancellationToken cancellationToken) => onlyIfCheap ? Task.FromResult(-1) : FillSetAsync(cancellationToken).ContinueWith(set => set.Result.Count);
+            public Task<int> GetCountAsync(bool onlyIfCheap, CancellationToken cancellationToken) => onlyIfCheap ? TaskExt.MinusOne : FillSetAsync(cancellationToken).ContinueWith(set => set.Result.Count);
         }
 
         /// <summary>