Introduce {ConstantElementsPair} struct for type safety.

src/full-codegen/s390/full-codegen-s390.cc

Index: src/full-codegen/s390/full-codegen-s390.cc
diff --git a/src/full-codegen/s390/full-codegen-s390.cc b/src/full-codegen/s390/full-codegen-s390.cc
index 2a96d293a7932af229a635b147d915adf026aa60..a912c294bbc7cbed8f1d8b311c92451e58670c48 100644
--- a/src/full-codegen/s390/full-codegen-s390.cc
+++ b/src/full-codegen/s390/full-codegen-s390.cc
@@ -1264,11 +1264,9 @@ 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_fast_elements =
       IsFastObjectElementsKind(expr->constant_elements_kind());
-  Handle<FixedArrayBase> constant_elements_values(
-      FixedArrayBase::cast(constant_elements->get(1)));
 
   AllocationSiteMode allocation_site_mode = TRACK_ALLOCATION_SITE;
   if (has_fast_elements && !FLAG_allocation_site_pretenuring) {