MIR tidying.

Fix memory leak in the MIR graph m_units_.
Don't shadow the arena_ field in Backend.

Change-Id: I0ffaebd088f1383205bde9fb34c5d33156bb7260
diff --git a/src/compiler/dex/mir_graph.h b/src/compiler/dex/mir_graph.h
index 6a45ac0..fd81967 100644
--- a/src/compiler/dex/mir_graph.h
+++ b/src/compiler/dex/mir_graph.h
@@ -310,7 +310,7 @@
 class MIRGraph {
  public:
   MIRGraph(CompilationUnit* cu, ArenaAllocator* arena);
-  ~MIRGraph() {}
+  ~MIRGraph();
 
   /*
    * Parse dex method and add MIR at current insert point.  Returns id (which is