Special handling for inline caches in code serializer.

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index 37ab88d95aded9e95aa72f258336115a6ab35885..b7c4591f2f4785a941ec3d6fc13dc969fab3403c 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -5132,6 +5132,10 @@ class Code: public HeapObject {
   inline void set_profiler_ticks(int ticks);
 
   // [builtin_index]: For BUILTIN kind, tells which builtin index it has.
+  // For builtins, tells which builtin index it has.
+  // Note that builtins can have a code kind other than BUILTIN, which means
+  // that for arbitrary code objects, this index value may be random garbage.
+  // To verify in that case, compare the code object to the indexed builtin.
   inline int builtin_index();
   inline void set_builtin_index(int id);