Replace HeapNumber as doublebox with an explicit MutableHeapNumber.

src/deoptimizer.cc

Index: src/deoptimizer.cc
diff --git a/src/deoptimizer.cc b/src/deoptimizer.cc
index 4d5e60573d29fd2aee427642c4e609de6ad2b03d..2528b26ec9a2804ae02771ef8edf0a3b2aa23880 100644
--- a/src/deoptimizer.cc
+++ b/src/deoptimizer.cc
@@ -1842,9 +1842,11 @@ Handle<Object> Deoptimizer::MaterializeNextHeapObject() {
     Handle<Map> map = Map::GeneralizeAllFieldRepresentations(
         Handle<Map>::cast(MaterializeNextValue()), Representation::Tagged());
     switch (map->instance_type()) {
+      case MUTABLE_HEAP_NUMBER_TYPE:
       case HEAP_NUMBER_TYPE: {
         // Reuse the HeapNumber value directly as it is already properly
-        // tagged and skip materializing the HeapNumber explicitly.
+        // tagged and skip materializing the HeapNumber explicitly. Turn mutable
+        // heap numbers immutable.
         Handle<Object> object = MaterializeNextValue();
         if (object_index < prev_materialized_count_) {
           materialized_objects_->Add(Handle<Object>(
@@ -1905,6 +1907,9 @@ Handle<Object> Deoptimizer::MaterializeNextHeapObject() {
 Handle<Object> Deoptimizer::MaterializeNextValue() {
   int value_index = materialization_value_index_++;
   Handle<Object> value = materialized_values_->at(value_index);
+  if (value->IsMutableHeapNumber()) {
+    HeapNumber::cast(*value)->set_map(isolate_->heap()->heap_number_map());
+  }
   if (*value == isolate_->heap()->arguments_marker()) {
     value = MaterializeNextHeapObject();
   }
@@ -3346,6 +3351,7 @@ Handle<Object> SlotRefValueBuilder::GetNext(Isolate* isolate, int lvl) {
       // TODO(jarin) this should be unified with the code in
       // Deoptimizer::MaterializeNextHeapObject()
       switch (map->instance_type()) {
+        case MUTABLE_HEAP_NUMBER_TYPE:
         case HEAP_NUMBER_TYPE: {
           // Reuse the HeapNumber value directly as it is already properly
           // tagged and skip materializing the HeapNumber explicitly.