[compiler] Properly guard the speculative optimizations for instanceof.

src/full-codegen/full-codegen.h

Index: src/full-codegen/full-codegen.h
diff --git a/src/full-codegen/full-codegen.h b/src/full-codegen/full-codegen.h
index aa1caaff98603834e94192e2a4712c2a5a5cbed0..617e5268baa0cc0a1c231dd5520fb56ce488b376 100644
--- a/src/full-codegen/full-codegen.h
+++ b/src/full-codegen/full-codegen.h
@@ -411,11 +411,15 @@ class FullCodeGenerator final : public AstVisitor<FullCodeGenerator> {
   void PrepareForBailout(Expression* node, Deoptimizer::BailoutState state);
   void PrepareForBailoutForId(BailoutId id, Deoptimizer::BailoutState state);
 
+  // Returns the plain index for the FixedArray that backs the feedback vector.
+  int SlotToIndex(FeedbackVectorSlot slot) const {
+    return TypeFeedbackVector::GetIndexFromSpec(
+        literal()->feedback_vector_spec(), slot);
+  }
   // Returns a smi for the index into the FixedArray that backs the feedback
   // vector
   Smi* SmiFromSlot(FeedbackVectorSlot slot) const {
-    return Smi::FromInt(TypeFeedbackVector::GetIndexFromSpec(
-        literal()->feedback_vector_spec(), slot));
+    return Smi::FromInt(SlotToIndex(slot));
   }
 
   // Record a call's return site offset, used to rebuild the frame if the