[turbofan] Cache Call operator for AllocateHeapNumberStub in change lowering.

src/compiler/change-lowering.h

Index: src/compiler/change-lowering.h
diff --git a/src/compiler/change-lowering.h b/src/compiler/change-lowering.h
index 3f986399372b7bbfd02578a1ecf93c0ce10fccf2..0d498ccd9760b10d8cfff279f20bacf169f6b4ce 100644
--- a/src/compiler/change-lowering.h
+++ b/src/compiler/change-lowering.h
@@ -16,6 +16,7 @@ class CommonOperatorBuilder;
 class JSGraph;
 class Linkage;
 class MachineOperatorBuilder;
+class Operator;
 
 class ChangeLowering FINAL : public Reducer {
  public:
@@ -54,7 +55,8 @@ class ChangeLowering FINAL : public Reducer {
   CommonOperatorBuilder* common() const;
   MachineOperatorBuilder* machine() const;
 
-  JSGraph* jsgraph_;
+  JSGraph* const jsgraph_;
+  SetOncePointer<const Operator> allocate_heap_number_operator_;
 };
 
 }  // namespace compiler