[turbofan] Properly unlink dead nodes in simplified lowering.

test/cctest/compiler/test-simplified-lowering.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 <limits>
 
 #include "src/compiler/access-builder.h"
 #include "src/compiler/change-lowering.h"
 #include "src/compiler/control-builders.h"
 #include "src/compiler/graph-reducer.h"
@@ skipping 1920 matching lines @@
 
 
 TEST(NumberModulus_TruncatingToUint32) {
   double constants[] = {1, 3, 100, 1000, 100998348};
 
   for (size_t i = 0; i < arraysize(constants); i++) {
     TestingGraph t(Type::Unsigned32());
     Node* k = t.jsgraph.Constant(constants[i]);
     Node* mod = t.graph()->NewNode(t.simplified()->NumberModulus(), t.p0, k);
     Node* trunc = t.graph()->NewNode(t.simplified()->NumberToUint32(), mod);
-    Node* ret = t.Return(trunc);
+    t.Return(trunc);
     t.Lower();
 
-    CHECK_EQ(IrOpcode::kUint32Mod, ret->InputAt(0)->opcode());
+    CHECK_EQ(IrOpcode::kUint32Mod, t.ret->InputAt(0)->InputAt(0)->opcode());
   }
 }
 
 
 TEST(RunNumberModulus_TruncatingToUint32) {
   uint32_t constants[] = {1, 2, 100, 1000, 1024, 2048};
 
   for (size_t i = 0; i < arraysize(constants); i++) {
     uint32_t k = constants[i];
     SimplifiedLoweringTester<Object*> t(kMachAnyTagged);
@@ skipping 85 matching lines @@
     Bounds phi_bounds = Bounds::Either(Bounds(d.arg1), Bounds(d.arg2), z);
     NodeProperties::SetBounds(phi, phi_bounds);
 
     Node* use = t.Use(phi, d.use);
     t.Return(use);
     t.Lower();
 
     CHECK_EQ(d.expected, OpParameter<MachineType>(phi));
   }
 }