[compiler] Remove obsolete JSYield operator.

src/compiler/ast-graph-builder.cc

 // Copyright 2014 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/compiler/ast-graph-builder.h"
 
 #include "src/ast/scopes.h"
 #include "src/compiler.h"
 #include "src/compiler/ast-loop-assignment-analyzer.h"
 #include "src/compiler/control-builders.h"
@@ skipping 2233 matching lines @@
                              ast_context()->GetStateCombine());
       break;
     }
   }
 
   ast_context()->ProduceValue(value);
 }
 
 
 void AstGraphBuilder::VisitYield(Yield* expr) {
-  // TODO(turbofan): Implement yield here.
+  // Generator functions are supported only by going through Ignition first.
   SetStackOverflow();
   ast_context()->ProduceValue(jsgraph()->UndefinedConstant());
 }
 
 
 void AstGraphBuilder::VisitThrow(Throw* expr) {
   VisitForValue(expr->exception());
   Node* exception = environment()->Pop();
   Node* value = BuildThrowError(exception, expr->id());
   ast_context()->ProduceValue(value);
@@ skipping 2093 matching lines @@
     // Phi does not exist yet, introduce one.
     value = NewPhi(inputs, value, control);
     value->ReplaceInput(inputs - 1, other);
   }
   return value;
 }
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8