Introduce {ConstantElementsPair} struct for type safety.

src/full-codegen/x64/full-codegen-x64.cc

Index: src/full-codegen/x64/full-codegen-x64.cc
diff --git a/src/full-codegen/x64/full-codegen-x64.cc b/src/full-codegen/x64/full-codegen-x64.cc
index 6c4048e7d28bf7aed6dc0fe988eb2108d0d189f0..79830c3d31620154b0280aea25bcc764e80764ee 100644
--- a/src/full-codegen/x64/full-codegen-x64.cc
+++ b/src/full-codegen/x64/full-codegen-x64.cc
@@ -1285,7 +1285,7 @@ void FullCodeGenerator::VisitObjectLiteral(ObjectLiteral* expr) {
 void FullCodeGenerator::VisitArrayLiteral(ArrayLiteral* expr) {
   Comment cmnt(masm_, "[ ArrayLiteral");
 
-  Handle<FixedArray> constant_elements = expr->constant_elements();
+  Handle<ConstantElementsPair> constant_elements = expr->constant_elements();
   bool has_constant_fast_elements =
       IsFastObjectElementsKind(expr->constant_elements_kind());