Array.FindAll improvements (#120336)

Now a small amount of matches are stack allocated and the array is only optionally allocated.

---------

Co-authored-by: Tarek Mahmoud Sayed <10833894+tarekgh@users.noreply.github.com>
Co-authored-by: Jan Kotas <jkotas@microsoft.com>

Author: 3 people

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

@@ -1553,15 +1553,27 @@ public static T[] FindAll<T>(T[] array, Predicate<T> match)
                 ThrowHelper.ThrowArgumentNullException(ExceptionArgument.match);
             }
 
-            List<T> list = new List<T>();
-            for (int i = 0; i < array.Length; i++)
+            InlineArray4<T> stackAllocatedMatches = default;
+            Span<T> span = stackAllocatedMatches;
+            int foundCount = 0;
+            T[]? values = null;
+
+            foreach (T value in array)
             {
-                if (match(array[i]))
+                if (match(value))
                 {
-                    list.Add(array[i]);
+                    if (foundCount >= span.Length)
+                    {
+                        values = new T[Math.Min((uint)span.Length * 2, (uint)array.Length)];
+                        span.CopyTo(values);
+                        span = values;
+                    }
+
+                    span[foundCount++] = value;
                 }
             }
-            return list.ToArray();
+
+            return values?.Length == foundCount ? values : span[..foundCount].ToArray();
         }
 
         public static int FindIndex<T>(T[] array, Predicate<T> match)