Avoid setting / depending on flags when transitioning, and when fields are known to be constant.

src/hydrogen-instructions.h

Index: src/hydrogen-instructions.h
diff --git a/src/hydrogen-instructions.h b/src/hydrogen-instructions.h
index d5b1c75ebf8b7f87bffd705e7c3a2dce999229ed..115264022fdff9bf96e61369598b6a03af7be106 100644
--- a/src/hydrogen-instructions.h
+++ b/src/hydrogen-instructions.h
@@ -5420,6 +5420,11 @@ class HStoreContextSlot V8_FINAL : public HTemplateInstruction<2> {
 // array elements pointer, etc, but not accesses to array elements themselves.
 class HObjectAccess V8_FINAL {
  public:
+  // Indicates whether the field is initialized while transitioning, or also
+  // mutable afterwards. Transitioning stores only initialize new fields rather
+  // than changing existing fields, so they are marked as CONSTANT.
+  enum FieldType { MUTABLE, CONSTANT };
+
   inline bool IsInobject() const {
     return portion() != kBackingStore && portion() != kExternalMemory;
   }
@@ -5444,8 +5449,10 @@ class HObjectAccess V8_FINAL {
     return name_;
   }
 
-  inline HObjectAccess WithRepresentation(Representation representation) {
-    return HObjectAccess(portion(), offset(), representation, name());
+  inline HObjectAccess WithRepresentation(Representation representation,
+                                          FieldType field_type = MUTABLE) {
+    return HObjectAccess(
+        portion(), offset(), representation, name(), field_type);
   }
 
   static HObjectAccess ForHeapNumberValue() {
@@ -5566,10 +5573,12 @@ class HObjectAccess V8_FINAL {
 
   HObjectAccess(Portion portion, int offset,
                 Representation representation = Representation::Tagged(),
-                Handle<String> name = Handle<String>::null())
+                Handle<String> name = Handle<String>::null(),
+                FieldType field_type = MUTABLE)
     : value_(PortionField::encode(portion) |
-             RepresentationField::encode(representation.kind()) |
-             OffsetField::encode(offset)),
+                RepresentationField::encode(representation.kind()) |
+                FieldTypeField::encode(field_type) |
+                OffsetField::encode(offset)),
       name_(name) {
     // assert that the fields decode correctly
     ASSERT(this->offset() == offset);
@@ -5579,7 +5588,8 @@ class HObjectAccess V8_FINAL {
 
   class PortionField : public BitField<Portion, 0, 3> {};
   class RepresentationField : public BitField<Representation::Kind, 3, 3> {};
-  class OffsetField : public BitField<int, 6, 26> {};
+  class FieldTypeField : public BitField<FieldType, 6, 1> {};
+  class OffsetField : public BitField<int, 7, 25> {};
 
   uint32_t value_;  // encodes portion, representation, and offset
   Handle<String> name_;