Introduce {ConstantElementsPair} struct for type safety.

src/full-codegen/mips64/full-codegen-mips64.cc

Index: src/full-codegen/mips64/full-codegen-mips64.cc
diff --git a/src/full-codegen/mips64/full-codegen-mips64.cc b/src/full-codegen/mips64/full-codegen-mips64.cc
index 8f289a59efea7e849a9c4f308641c6fe1bf7321f..af2ce9d85162d5d53507d1f20ab95bde14769b3c 100644
--- a/src/full-codegen/mips64/full-codegen-mips64.cc
+++ b/src/full-codegen/mips64/full-codegen-mips64.cc
@@ -1334,7 +1334,7 @@ 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_fast_elements =
       IsFastObjectElementsKind(expr->constant_elements_kind());