Introduce {ConstantElementsPair} struct for type safety.

test/cctest/interpreter/bytecode_expectations/ForIn.golden

 #
 # Autogenerated by generate-bytecode-expectations.
 #
 
 ---
 wrap: yes
 
 ---
 snippet: "
   for (var p in null) {}
@@ skipping 114 matching lines @@
   /*   75 E> */ B(Add), R(0), U8(2),
                 B(Mov), R(0), R(8),
                 B(Star), R(0),
   /*   72 E> */ B(ForInStep), R(7),
                 B(Star), R(7),
                 B(JumpLoop), U8(-31), U8(0),
                 B(LdaUndefined),
   /*   80 S> */ B(Return),
 ]
 constant pool: [
-  FIXED_ARRAY_TYPE,
+  CONSTANT_ELEMENTS_PAIR_TYPE,
 ]
 handlers: [
 ]
 
 ---
 snippet: "
   var x = { 'a': 1, 'b': 2 };
   for (x['a'] in [10, 20, 30]) {
     if (x['a'] == 10) continue;
     if (x['a'] == 20) break;
@@ skipping 35 matching lines @@
                 B(JumpIfFalse), U8(4),
   /*  143 S> */ B(Jump), U8(9),
                 B(ForInStep), R(5),
                 B(Star), R(5),
                 B(JumpLoop), U8(-55), U8(0),
                 B(LdaUndefined),
   /*  152 S> */ B(Return),
 ]
 constant pool: [
   FIXED_ARRAY_TYPE,
-  FIXED_ARRAY_TYPE,
+  CONSTANT_ELEMENTS_PAIR_TYPE,
   ONE_BYTE_INTERNALIZED_STRING_TYPE ["a"],
 ]
 handlers: [
 ]
 
 ---
 snippet: "
   var x = [ 10, 11, 12 ] ;
   for (x[0] in [1,2,3]) { return x[3]; }
 "
@@ skipping 24 matching lines @@
   /*   83 S> */ B(LdaSmi), U8(3),
   /*   91 E> */ B(LdaKeyedProperty), R(0), U8(4),
   /*   98 S> */ B(Return),
                 B(ForInStep), R(5),
                 B(Star), R(5),
                 B(JumpLoop), U8(-34), U8(0),
                 B(LdaUndefined),
   /*   98 S> */ B(Return),
 ]
 constant pool: [
-  FIXED_ARRAY_TYPE,
-  FIXED_ARRAY_TYPE,
+  CONSTANT_ELEMENTS_PAIR_TYPE,
+  CONSTANT_ELEMENTS_PAIR_TYPE,
 ]
 handlers: [
 ]