[turbofan] Check for dead node in the common operator reducer.

src/compiler/common-operator-reducer.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/common-operator-reducer.h"
 
 #include <algorithm>
 
 #include "src/compiler/common-operator.h"
 #include "src/compiler/graph.h"
@@ skipping 186 matching lines @@
     Node* if_true = merge->InputAt(0);
     Node* if_false = merge->InputAt(1);
     if (if_true->opcode() != IrOpcode::kIfTrue) {
       std::swap(if_true, if_false);
       std::swap(vtrue, vfalse);
     }
     if (if_true->opcode() == IrOpcode::kIfTrue &&
         if_false->opcode() == IrOpcode::kIfFalse &&
         if_true->InputAt(0) == if_false->InputAt(0)) {
       Node* const branch = if_true->InputAt(0);
+      // Check that the branch is not dead already.
+      if (branch->opcode() != IrOpcode::kBranch) return NoChange();
       Node* const cond = branch->InputAt(0);
       if (cond->opcode() == IrOpcode::kFloat32LessThan) {
         Float32BinopMatcher mcond(cond);
         if (mcond.left().Is(0.0) && mcond.right().Equals(vtrue) &&
             vfalse->opcode() == IrOpcode::kFloat32Sub) {
           Float32BinopMatcher mvfalse(vfalse);
           if (mvfalse.left().IsZero() && mvfalse.right().Equals(vtrue)) {
             // We might now be able to further reduce the {merge} node.
             Revisit(merge);
             return Change(node, machine()->Float32Abs(), vtrue);
@@ skipping 167 matching lines @@
   node->ReplaceInput(0, a);
   node->ReplaceInput(1, b);
   node->TrimInputCount(2);
   NodeProperties::ChangeOp(node, op);
   return Changed(node);
 }
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8