Simplify and compact transitions storage

src/heap/heap.cc

Index: src/heap/heap.cc
diff --git a/src/heap/heap.cc b/src/heap/heap.cc
index 26cffc8fb5524e51310776f74c2fbc3e0e7edf8b..148b1ccfa572afbff1a5751ac9624c08d8a52931 100644
--- a/src/heap/heap.cc
+++ b/src/heap/heap.cc
@@ -2473,7 +2473,7 @@ AllocationResult Heap::AllocateMap(InstanceType instance_type,
   map->set_dependent_code(DependentCode::cast(empty_fixed_array()),
                           SKIP_WRITE_BARRIER);
   map->set_weak_cell_cache(Smi::FromInt(0));
-  map->init_transitions(undefined_value());
+  map->set_raw_transitions(Smi::FromInt(0));
   map->set_unused_property_fields(0);
   map->set_instance_descriptors(empty_descriptor_array());
   if (FLAG_unbox_double_fields) {
@@ -2607,7 +2607,7 @@ bool Heap::CreateInitialMaps() {
   // Fix the instance_descriptors for the existing maps.
   meta_map()->set_code_cache(empty_fixed_array());
   meta_map()->set_dependent_code(DependentCode::cast(empty_fixed_array()));
-  meta_map()->init_transitions(undefined_value());
+  meta_map()->set_raw_transitions(Smi::FromInt(0));
   meta_map()->set_instance_descriptors(empty_descriptor_array());
   if (FLAG_unbox_double_fields) {
     meta_map()->set_layout_descriptor(LayoutDescriptor::FastPointerLayout());
@@ -2616,7 +2616,7 @@ bool Heap::CreateInitialMaps() {
   fixed_array_map()->set_code_cache(empty_fixed_array());
   fixed_array_map()->set_dependent_code(
       DependentCode::cast(empty_fixed_array()));
-  fixed_array_map()->init_transitions(undefined_value());
+  fixed_array_map()->set_raw_transitions(Smi::FromInt(0));
   fixed_array_map()->set_instance_descriptors(empty_descriptor_array());
   if (FLAG_unbox_double_fields) {
     fixed_array_map()->set_layout_descriptor(
@@ -2625,7 +2625,7 @@ bool Heap::CreateInitialMaps() {
 
   undefined_map()->set_code_cache(empty_fixed_array());
   undefined_map()->set_dependent_code(DependentCode::cast(empty_fixed_array()));
-  undefined_map()->init_transitions(undefined_value());
+  undefined_map()->set_raw_transitions(Smi::FromInt(0));
   undefined_map()->set_instance_descriptors(empty_descriptor_array());
   if (FLAG_unbox_double_fields) {
     undefined_map()->set_layout_descriptor(
@@ -2634,7 +2634,7 @@ bool Heap::CreateInitialMaps() {
 
   null_map()->set_code_cache(empty_fixed_array());
   null_map()->set_dependent_code(DependentCode::cast(empty_fixed_array()));
-  null_map()->init_transitions(undefined_value());
+  null_map()->set_raw_transitions(Smi::FromInt(0));
   null_map()->set_instance_descriptors(empty_descriptor_array());
   if (FLAG_unbox_double_fields) {
     null_map()->set_layout_descriptor(LayoutDescriptor::FastPointerLayout());
@@ -2643,7 +2643,7 @@ bool Heap::CreateInitialMaps() {
   constant_pool_array_map()->set_code_cache(empty_fixed_array());
   constant_pool_array_map()->set_dependent_code(
       DependentCode::cast(empty_fixed_array()));
-  constant_pool_array_map()->init_transitions(undefined_value());
+  constant_pool_array_map()->set_raw_transitions(Smi::FromInt(0));
   constant_pool_array_map()->set_instance_descriptors(empty_descriptor_array());
   if (FLAG_unbox_double_fields) {
     constant_pool_array_map()->set_layout_descriptor(