Make Queue.Peek inline (dotnet/corefx#12062)

Commit migrated from dotnet/corefx@2d0e2ab

Author: jamesqo

src/libraries/System.Collections/src/System/Collections/Generic/Queue.cs

@@ -228,7 +228,9 @@ IEnumerator IEnumerable.GetEnumerator()
         public T Dequeue()
         {
             if (_size == 0)
-                throw new InvalidOperationException(SR.InvalidOperation_EmptyQueue);
+            {
+                ThrowForEmptyQueue();
+            }
 
             T removed = _array[_head];
             _array[_head] = default(T);
@@ -244,8 +246,10 @@ public T Dequeue()
         public T Peek()
         {
             if (_size == 0)
-                throw new InvalidOperationException(SR.InvalidOperation_EmptyQueue);
-
+            {
+                ThrowForEmptyQueue();
+            }
+            
             return _array[_head];
         }
 
@@ -328,6 +332,12 @@ private void MoveNext(ref int index)
             index = (tmp == _array.Length) ? 0 : tmp;
         }
 
+        private void ThrowForEmptyQueue()
+        {
+            Debug.Assert(_size == 0);
+            throw new InvalidOperationException(SR.InvalidOperation_EmptyQueue);
+        }
+
         public void TrimExcess()
         {
             int threshold = (int)(((double)_array.Length) * 0.9);