Introduce {ConstantElementsPair} struct for type safety.

src/ast/ast.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_AST_AST_H_
 #define V8_AST_AST_H_
 
 #include "src/ast/ast-types.h"
 #include "src/ast/ast-value-factory.h"
 #include "src/ast/modules.h"
@@ skipping 1519 matching lines @@
 
   int const flags_;
   const AstRawString* const pattern_;
 };
 
 
 // An array literal has a literals object that is used
 // for minimizing the work when constructing it at runtime.
 class ArrayLiteral final : public MaterializedLiteral {
  public:
-  Handle<FixedArray> constant_elements() const { return constant_elements_; }
+  Handle<ConstantElementsPair> constant_elements() const {
+    return constant_elements_;
+  }
   ElementsKind constant_elements_kind() const {
-    DCHECK_EQ(2, constant_elements_->length());
-    return static_cast<ElementsKind>(
-        Smi::cast(constant_elements_->get(0))->value());
+    return static_cast<ElementsKind>(constant_elements()->elements_kind());
   }
 
   ZoneList<Expression*>* values() const { return values_; }
 
   BailoutId CreateLiteralId() const { return BailoutId(local_id(0)); }
 
   // Return an AST id for an element that is used in simulate instructions.
   BailoutId GetIdForElement(int i) { return BailoutId(local_id(i + 1)); }
 
   // Unlike other AST nodes, this number of bailout IDs allocated for an
@@ skipping 42 matching lines @@
                int literal_index, int pos)
       : MaterializedLiteral(literal_index, pos, kArrayLiteral),
         first_spread_index_(first_spread_index),
         values_(values) {}
 
   static int parent_num_ids() { return MaterializedLiteral::num_ids(); }
   int local_id(int n) const { return base_id() + parent_num_ids() + n; }
 
   int first_spread_index_;
   FeedbackVectorSlot literal_slot_;
-  Handle<FixedArray> constant_elements_;
+  Handle<ConstantElementsPair> constant_elements_;
   ZoneList<Expression*>* values_;
 };
 
 
 class VariableProxy final : public Expression {
  public:
   bool IsValidReferenceExpression() const {
     return !is_this() && !is_new_target();
   }
 
@@ skipping 2007 matching lines @@
                                     : NULL;                                   \
   }
 AST_NODE_LIST(DECLARE_NODE_FUNCTIONS)
 #undef DECLARE_NODE_FUNCTIONS
 
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_AST_AST_H_