ART: Mterp read barrier fix + minor cleanup

Read barrier support relies on hooks in common code for loading
object references.  Mterp missed doing this for iget-object-quick.
Also, added missing conditional assembly around debug event logging
for mterp fallback and deleted an unnecessary store.

Bug: 26510411

Change-Id: I2d5b27c4090be58d3cfcb14309d14ccabf04a6f5
diff --git a/runtime/interpreter/mterp/mterp.cc b/runtime/interpreter/mterp/mterp.cc
index 060fe76..9975458 100644
--- a/runtime/interpreter/mterp/mterp.cc
+++ b/runtime/interpreter/mterp/mterp.cc
@@ -607,5 +607,14 @@
   }
 }
 
+extern "C" mirror::Object* artIGetObjectFromMterp(mirror::Object* obj, uint32_t field_offset)
+  SHARED_REQUIRES(Locks::mutator_lock_) {
+  if (UNLIKELY(obj == nullptr)) {
+    ThrowNullPointerExceptionFromInterpreter();
+    return nullptr;
+  }
+  return obj->GetFieldObject<mirror::Object>(MemberOffset(field_offset));
+}
+
 }  // namespace interpreter
 }  // namespace art