[turbofan] Make Node::set_op safer via wrapper.

src/compiler/node.h

Index: src/compiler/node.h
diff --git a/src/compiler/node.h b/src/compiler/node.h
index bd129653bc3397510a53169c0ce48ae9ef411004..d6a9b39a56419b4682f1234234b9211e60563418 100644
--- a/src/compiler/node.h
+++ b/src/compiler/node.h
@@ -49,7 +49,6 @@ class Node final {
   void Kill();
 
   const Operator* op() const { return op_; }
-  void set_op(const Operator* op) { op_ = op; }
 
   IrOpcode::Value opcode() const {
     DCHECK(op_->opcode() <= IrOpcode::kLast);
@@ -284,6 +283,9 @@ class Node final {
 
   void* operator new(size_t, void* location) { return location; }
 
+  // Only NodeProperties should manipulate the op.
+  void set_op(const Operator* op) { op_ = op; }
+
   // Only NodeProperties should manipulate the type.
   Type* type() const { return type_; }
   void set_type(Type* type) { type_ = type; }