Add fast paths to WhenAny(IEnumerable) (#38896)

* Add fast paths to WhenAny(IEnumerable)

Adds fast paths in case `tasks` is an array or an `ICollection`

* Remove cast from slow path for WhenAll and WhenAny

Author: Bond-009

src/libraries/System.Private.CoreLib/src/System/Threading/Tasks/Task.cs

@@ -5469,15 +5469,15 @@ protected override void Cleanup()
         /// </exception>
         public static Task WhenAll(IEnumerable<Task> tasks)
         {
-            // Take a more efficient path if tasks is actually an array
-            if (tasks is Task[] taskArray)
-            {
-                return WhenAll(taskArray);
-            }
-
             // Skip a List allocation/copy if tasks is a collection
             if (tasks is ICollection<Task> taskCollection)
             {
+                // Take a more efficient path if tasks is actually an array
+                if (tasks is Task[] taskArray)
+                {
+                    return WhenAll(taskArray);
+                }
+
                 int index = 0;
                 taskArray = new Task[taskCollection.Count];
                 foreach (Task task in tasks)
@@ -6077,6 +6077,25 @@ public void Invoke(Task completingTask)
         /// </exception>
         public static Task<Task> WhenAny(IEnumerable<Task> tasks)
         {
+            // Skip a List allocation/copy if tasks is a collection
+            if (tasks is ICollection<Task> taskCollection)
+            {
+                // Take a more efficient path if tasks is actually an array
+                if (tasks is Task[] taskArray)
+                {
+                    return WhenAny(taskArray);
+                }
+
+                int index = 0;
+                taskArray = new Task[taskCollection.Count];
+                foreach (Task task in tasks)
+                {
+                    if (task == null) ThrowHelper.ThrowArgumentException(ExceptionResource.Task_MultiTaskContinuation_NullTask, ExceptionArgument.tasks);
+                    taskArray[index++] = task;
+                }
+                return TaskFactory.CommonCWAnyLogic(taskArray);
+            }
+
             if (tasks == null) ThrowHelper.ThrowArgumentNullException(ExceptionArgument.tasks);
 
             // Make a defensive copy, as the user may manipulate the tasks collection