Introduce {ConstantElementsPair} struct for type safety.

test/cctest/interpreter/bytecode_expectations/ArrayLiterals.golden

 #
 # Autogenerated by generate-bytecode-expectations.
 #
 
 ---
 wrap: yes
 
 ---
 snippet: "
   return [ 1, 2 ];
 "
 frame size: 0
 parameter count: 1
 bytecode array length: 6
 bytecodes: [
   /*   30 E> */ B(StackCheck),
   /*   34 S> */ B(CreateArrayLiteral), U8(0), U8(0), U8(9),
   /*   51 S> */ B(Return),
 ]
 constant pool: [
-  FIXED_ARRAY_TYPE,
+  CONSTANT_ELEMENTS_PAIR_TYPE,
 ]
 handlers: [
 ]
 
 ---
 snippet: "
   var a = 1; return [ a, a + 1 ];
 "
 frame size: 3
 parameter count: 1
 bytecode array length: 35
 bytecodes: [
   /*   30 E> */ B(StackCheck),
   /*   42 S> */ B(LdaSmi), U8(1),
                 B(Star), R(0),
   /*   45 S> */ B(CreateArrayLiteral), U8(0), U8(0), U8(9),
                 B(Star), R(2),
                 B(LdaZero),
                 B(Star), R(1),
                 B(Ldar), R(0),
   /*   54 E> */ B(StaKeyedPropertySloppy), R(2), R(1), U8(3),
                 B(LdaSmi), U8(1),
                 B(Star), R(1),
   /*   59 E> */ B(AddSmi), U8(1), R(0), U8(2),
                 B(StaKeyedPropertySloppy), R(2), R(1), U8(3),
                 B(Ldar), R(2),
   /*   66 S> */ B(Return),
 ]
 constant pool: [
-  FIXED_ARRAY_TYPE,
+  CONSTANT_ELEMENTS_PAIR_TYPE,
 ]
 handlers: [
 ]
 
 ---
 snippet: "
   return [ [ 1, 2 ], [ 3 ] ];
 "
 frame size: 0
 parameter count: 1
 bytecode array length: 6
 bytecodes: [
   /*   30 E> */ B(StackCheck),
   /*   34 S> */ B(CreateArrayLiteral), U8(0), U8(2), U8(0),
   /*   62 S> */ B(Return),
 ]
 constant pool: [
-  FIXED_ARRAY_TYPE,
+  CONSTANT_ELEMENTS_PAIR_TYPE,
 ]
 handlers: [
 ]
 
 ---
 snippet: "
   var a = 1; return [ [ a, 2 ], [ a + 2 ] ];
 "
 frame size: 5
 parameter count: 1
@@ skipping 21 matching lines @@
                 B(LdaZero),
                 B(Star), R(3),
   /*   68 E> */ B(AddSmi), U8(2), R(0), U8(4),
                 B(StaKeyedPropertySloppy), R(4), R(3), U8(5),
                 B(Ldar), R(4),
                 B(StaKeyedPropertySloppy), R(2), R(1), U8(7),
                 B(Ldar), R(2),
   /*   77 S> */ B(Return),
 ]
 constant pool: [
-  FIXED_ARRAY_TYPE,
-  FIXED_ARRAY_TYPE,
-  FIXED_ARRAY_TYPE,
+  CONSTANT_ELEMENTS_PAIR_TYPE,
+  CONSTANT_ELEMENTS_PAIR_TYPE,
+  CONSTANT_ELEMENTS_PAIR_TYPE,
 ]
 handlers: [
 ]