Don't squash layers that have a clip or overflow clip.

LayoutTests/compositing/layer-creation/overflow-scroll-overlap-expected.txt

 (GraphicsLayer
   (bounds 800.00 600.00)
   (children 1
     (GraphicsLayer
       (bounds 800.00 600.00)
       (contentsOpaque 1)
       (drawsContent 1)
-      (children 2
+      (children 3
         (GraphicsLayer
-          (children 2
+          (position 8.00 8.00)
+          (bounds 30.00 30.00)
+        )
+        (GraphicsLayer
+          (position 20.00 20.00)
+          (bounds 306.00 206.00)
+          (drawsContent 1)
+          (children 1
             (GraphicsLayer
-              (position 8.00 8.00)
-              (bounds 30.00 30.00)
-            )
-            (GraphicsLayer
-              (position 20.00 20.00)
-              (bounds 306.00 206.00)
+              (position 288.00 3.00)
+              (bounds 15.00 200.00)
               (drawsContent 1)
             )
           )
         )
         (GraphicsLayer
           (position 23.00 23.00)
           (bounds 285.00 200.00)
           (children 1
             (GraphicsLayer
               (position 20.00 45.00)
               (bounds 210.00 100.00)
               (contentsOpaque 1)
               (drawsContent 1)
               (backgroundColor #0000FF)
             )
           )
         )
       )
     )
   )
 )