Move field index into property details, freeing up the value slot of fields.

src/objects-inl.h

Index: src/objects-inl.h
diff --git a/src/objects-inl.h b/src/objects-inl.h
index d0958eef29a0faa736980f8b8bd23887da101375..3d64383c46097e9200c3e65251005f7efb6df00b 100644
--- a/src/objects-inl.h
+++ b/src/objects-inl.h
@@ -58,7 +58,10 @@ PropertyDetails::PropertyDetails(Smi* smi) {
 
 
 Smi* PropertyDetails::AsSmi() {
-  return Smi::FromInt(value_);
+  // Ensure the upper 2 bits have the same value by sign extending it. This is
+  // necessary to be able to use the 31st bit of the property details.
+  int value = value_ << 1;
+  return Smi::FromInt(value >> 1);
 }
 
 
@@ -2327,7 +2330,7 @@ PropertyType DescriptorArray::GetType(int descriptor_number) {
 
 
 int DescriptorArray::GetFieldIndex(int descriptor_number) {
-  return Descriptor::IndexFromValue(GetValue(descriptor_number));
+  return GetDetails(descriptor_number).field_index();
 }