Reduce code size of Stack.Peek (#12094)

Author: jamesqo

src/System.Collections/src/System/Collections/Generic/Stack.cs

@@ -196,7 +196,10 @@ public void TrimExcess()
         public T Peek()
         {
             if (_size == 0)
-                throw new InvalidOperationException(SR.InvalidOperation_EmptyStack);
+            {
+                ThrowForEmptyStack();
+            }
+            
             return _array[_size - 1];
         }
 
@@ -216,7 +219,10 @@ public bool TryPeek(out T result)
         public T Pop()
         {
             if (_size == 0)
-                throw new InvalidOperationException(SR.InvalidOperation_EmptyStack);
+            {
+                ThrowForEmptyStack();
+            }
+            
             _version++;
             T item = _array[--_size];
             _array[_size] = default(T);     // Free memory quicker.
@@ -264,6 +270,12 @@ public T[] ToArray()
             return objArray;
         }
 
+        private void ThrowForEmptyStack()
+        {
+            Debug.Assert(_size == 0);
+            throw new InvalidOperationException(SR.InvalidOperation_EmptyStack);
+        }
+
         [SuppressMessage("Microsoft.Performance", "CA1815:OverrideEqualsAndOperatorEqualsOnValueTypes", Justification = "not an expected scenario")]
         [Serializable]
         public struct Enumerator : IEnumerator<T>, System.Collections.IEnumerator