[myanmar] Use buffer->sort() to sort cluster

This can possibly produce more granular clusters.
diff --git a/src/hb-ot-shape-complex-myanmar.cc b/src/hb-ot-shape-complex-myanmar.cc
index 9afcbf8..30a53c7 100644
--- a/src/hb-ot-shape-complex-myanmar.cc
+++ b/src/hb-ot-shape-complex-myanmar.cc
@@ -391,9 +391,8 @@
     }
   }
 
-  buffer->merge_clusters (start, end);
   /* Sit tight, rock 'n roll! */
-  hb_stable_sort (info + start, end - start, compare_myanmar_order);
+  buffer->sort (start, end, compare_myanmar_order);
 }
 
 static void