[literals] Don't reserve unused FeedbackVectorSlot for the empty object literal

Bug: v8:6211
Change-Id: Ie838cf118679e12483689e2c223e7ecc8335db18
Reviewed-on: https://chromium-review.googlesource.com/662759Reviewed-by: Marja Hölttä <marja@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48418}

src/ast/ast.cc

@@ -393,6 +393,9 @@ void ObjectLiteral::AssignFeedbackSlots(FeedbackVectorSpec* spec,
                                         LanguageMode language_mode,
                                         FunctionKind kind,
                                         FeedbackSlotCache* cache) {
+  // The empty object literal doesn't need any feedback vector slot.
+  if (this->IsEmptyObjectLiteral()) return;
+
   MaterializedLiteral::AssignFeedbackSlots(spec, language_mode, kind, cache);
 
   // This logic that computes the number of slots needed for vector store

test/cctest/interpreter/bytecode_expectations/ObjectLiterals.golden

@@ -338,10 +338,10 @@ bytecodes: [
   /*   30 E> */ B(StackCheck),
   /*   42 S> */ B(LdaConstant), U8(0),
                 B(Star), R(0),
-  /*   50 S> */ B(CreateObjectLiteral), U8(1), U8(1), U8(41), R(1),
+  /*   50 S> */ B(CreateObjectLiteral), U8(1), U8(0), U8(41), R(1),
   /*   60 E> */ B(ToName), R(2),
                 B(LdaSmi), I8(1),
-                B(StaDataPropertyInLiteral), R(1), R(2), U8(0), U8(2),
+                B(StaDataPropertyInLiteral), R(1), R(2), U8(0), U8(1),
                 B(CreateEmptyObjectLiteral),
                 B(Star), R(3),
                 B(Mov), R(1), R(2),