Move MethodHelper::GetReturnType to mirror::ArtMethod.

Also, fix missing handle in HasSameSignatureWithDifferentClassLoaders.

Change-Id: I9e1ffd09be950ecc8346fc3c485760d82d9ecab3
diff --git a/runtime/interpreter/interpreter_goto_table_impl.cc b/runtime/interpreter/interpreter_goto_table_impl.cc
index b970879..3a177eb 100644
--- a/runtime/interpreter/interpreter_goto_table_impl.cc
+++ b/runtime/interpreter/interpreter_goto_table_impl.cc
@@ -321,9 +321,7 @@
     const uint8_t vreg_index = inst->VRegA_11x(inst_data);
     Object* obj_result = shadow_frame.GetVRegReference(vreg_index);
     if (do_assignability_check && obj_result != NULL) {
-      StackHandleScope<1> hs(self);
-      MethodHelper mh(hs.NewHandle(shadow_frame.GetMethod()));
-      Class* return_type = mh.GetReturnType();
+      Class* return_type = shadow_frame.GetMethod()->GetReturnType();
       obj_result = shadow_frame.GetVRegReference(vreg_index);
       if (return_type == NULL) {
         // Return the pending exception.