Revert "Use IsAssignableFrom instead of IsSubclass for robustness."

Not correct for Quick.

Bug:21870666

This reverts commit af268bbc4fbd0b9847acf1f7b79c038806118467.

Change-Id: Id44849cc877a42495619fd71bfd2db75159b6868
diff --git a/compiler/driver/compiler_driver-inl.h b/compiler/driver/compiler_driver-inl.h
index 09ccd28..e0c56fc 100644
--- a/compiler/driver/compiler_driver-inl.h
+++ b/compiler/driver/compiler_driver-inl.h
@@ -250,7 +250,7 @@
 
 inline bool CompilerDriver::CanReferrerAssumeClassIsInitialized(mirror::Class* referrer_class,
                                                                 mirror::Class* klass) {
-  return (referrer_class != nullptr && klass->IsAssignableFrom(referrer_class)) ||
+  return (referrer_class != nullptr && referrer_class->IsSubClass(klass)) ||
       CanAssumeClassIsInitialized(klass);
 }