Reland "In-object double fields unboxing (for 64-bit only)."

src/field-index.h

Index: src/field-index.h
diff --git a/src/field-index.h b/src/field-index.h
index 2558529070b8a09a787208dc897486d0d4d69d27..76b111632d4bb39c236ba5bf8a50f540a80fa0f4 100644
--- a/src/field-index.h
+++ b/src/field-index.h
@@ -34,6 +34,8 @@ class FieldIndex FINAL {
     return IsInObjectBits::decode(bit_field_);
   }
 
+  bool is_hidden_field() const { return IsHiddenField::decode(bit_field_); }
+
   bool is_double() const {
     return IsDoubleBits::decode(bit_field_);
   }
@@ -55,7 +57,7 @@ class FieldIndex FINAL {
   // Zero-based from the first inobject property. Overflows to out-of-object
   // properties.
   int property_index() const {
-    DCHECK(!IsHiddenField::decode(bit_field_));
+    DCHECK(!is_hidden_field());
     int result = index() - first_inobject_property_offset() / kPointerSize;
     if (!is_inobject()) {
       result += InObjectPropertyBits::decode(bit_field_);
@@ -86,7 +88,7 @@ class FieldIndex FINAL {
   explicit FieldIndex(int bit_field) : bit_field_(bit_field) {}
 
   int first_inobject_property_offset() const {
-    DCHECK(!IsHiddenField::decode(bit_field_));
+    DCHECK(!is_hidden_field());
     return FirstInobjectPropertyOffsetBits::decode(bit_field_);
   }