[cleanup] Refactor builtins-number.cc

Introducing a TF_BUILTIN macro that wraps CodeStubAssembler usage
into a convenient interface (using a subclass under the hood).

Review-Url: https://codereview.chromium.org/2517833005
Cr-Commit-Position: refs/heads/master@{#41236}

src/builtins/builtins-utils.h

@@ -101,6 +101,31 @@ class BuiltinArguments : public Arguments {
   MUST_USE_RESULT static Object* Builtin_Impl_##name(BuiltinArguments args,   \
                                                      Isolate* isolate)
 
+// ----------------------------------------------------------------------------
+// Support macro for defining builtins with Turbofan.
+// ----------------------------------------------------------------------------
+//
+// A builtin function is defined by writing:
+//
+//   TF_BUILTIN(name, code_assember_base_class) {
+//     ...
+//   }
+//
+// In the body of the builtin function the arguments can be accessed
+// as "Parameter(n)".
+#define TF_BUILTIN(Name, AssemblerBase)                                 \
+  class Name##Assembler : public AssemblerBase {                        \
+   public:                                                              \
+    explicit Name##Assembler(compiler::CodeAssemblerState* state)       \
+        : AssemblerBase(state) {}                                       \
+    void Generate##Name();                                              \
+  };                                                                    \
+  void Builtins::Generate_##Name(compiler::CodeAssemblerState* state) { \
+    Name##Assembler assembler(state);                                   \
+    assembler.Generate##Name();                                         \
+  }                                                                     \
+  void Name##Assembler::Generate##Name()
+
 // ----------------------------------------------------------------------------
 
 #define CHECK_RECEIVER(Type, name, method)                                  \