Revert "Fix the computation of linear ordering."

Build is broken.

This reverts commit 3054a90063d379ab8c9e5a42a7daf0d644b48b07.

Change-Id: I259bc2bd6a58e30391b8176f3db5fdb5c07e4d6d
diff --git a/compiler/optimizing/nodes.h b/compiler/optimizing/nodes.h
index 19cd120..7d52d7d 100644
--- a/compiler/optimizing/nodes.h
+++ b/compiler/optimizing/nodes.h
@@ -233,7 +233,7 @@
     return false;
   }
 
-  size_t NumberOfBackEdges() const {
+  int NumberOfBackEdges() const {
     return back_edges_.Size();
   }