Fix compilation with verify heap on in release mode

BUG=none
TBR=rossberg@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/788603002

Cr-Commit-Position: refs/heads/master@{#25702}

src/objects-inl.h

@@ -6536,7 +6536,7 @@ void JSRegExp::SetDataAt(int index, Object* value) {
 
 ElementsKind JSObject::GetElementsKind() {
   ElementsKind kind = map()->elements_kind();
-#if VERIFY_HEAP
+#if VERIFY_HEAP && DEBUG
   FixedArrayBase* fixed_array =
       reinterpret_cast<FixedArrayBase*>(READ_FIELD(this, kElementsOffset));