ART: Use ObjPtr for GetSuperClass

Bug: 37224696
Test: mmma art
Test: m test-art-host
Change-Id: If55db6e26bb1bb05a13c59b849bed17839fca6cf
diff --git a/compiler/driver/compiler_driver.cc b/compiler/driver/compiler_driver.cc
index c5416d5..a32de29 100644
--- a/compiler/driver/compiler_driver.cc
+++ b/compiler/driver/compiler_driver.cc
@@ -1146,7 +1146,7 @@
     if (klass->IsArrayClass()) {
       MaybeAddToImageClasses(self, klass->GetComponentType(), image_classes);
     }
-    klass.Assign(klass->GetSuperClass());
+    klass = klass->GetSuperClass();
   }
 }