Use mutable heapnumbers to store doubles in fields.

src/x64/lithium-x64.h

Index: src/x64/lithium-x64.h
diff --git a/src/x64/lithium-x64.h b/src/x64/lithium-x64.h
index ccfb5b1dfb360db4ce9e5ef27c66019e64e5a22a..da61f32f810f84976d00e4627b917869d6fd91a5 100644
--- a/src/x64/lithium-x64.h
+++ b/src/x64/lithium-x64.h
@@ -1458,15 +1458,13 @@ class LReturn: public LTemplateInstruction<0, 2, 0> {
 };
 
 
-class LLoadNamedField: public LTemplateInstruction<1, 1, 1> {
+class LLoadNamedField: public LTemplateInstruction<1, 1, 0> {
  public:
-  explicit LLoadNamedField(LOperand* object, LOperand* temp) {
+  explicit LLoadNamedField(LOperand* object) {
     inputs_[0] = object;
-    temps_[0] = temp;
   }
 
   LOperand* object() { return inputs_[0]; }
-  LOperand* temp() { return temps_[0]; }
 
   DECLARE_CONCRETE_INSTRUCTION(LoadNamedField, "load-named-field")
   DECLARE_HYDROGEN_ACCESSOR(LoadNamedField)