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/driver/dex_compilation_unit.h b/src/compiler/driver/dex_compilation_unit.h
index 0fc1123..a9bb64f 100644
--- a/src/compiler/driver/dex_compilation_unit.h
+++ b/src/compiler/driver/dex_compilation_unit.h
@@ -97,7 +97,7 @@
   }
 
  private:
-  CompilationUnit* cu_;
+  CompilationUnit* const cu_;
 
   const jobject class_loader_;