[turbofan] Enable typed lowering of string addition.

src/compiler/verifier.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/verifier.h"
 
 #include <algorithm>
 #include <deque>
 #include <queue>
 #include <sstream>
@@ skipping 626 matching lines @@
       CheckUpperIs(node, Type::Number());
       break;
     case IrOpcode::kStringEqual:
     case IrOpcode::kStringLessThan:
     case IrOpcode::kStringLessThanOrEqual:
       // (String, String) -> Boolean
       CheckValueInputIs(node, 0, Type::String());
       CheckValueInputIs(node, 1, Type::String());
       CheckUpperIs(node, Type::Boolean());
       break;
-    case IrOpcode::kStringAdd:
-      // (String, String) -> String
-      CheckValueInputIs(node, 0, Type::String());
-      CheckValueInputIs(node, 1, Type::String());
-      CheckUpperIs(node, Type::String());
-      break;
     case IrOpcode::kReferenceEqual: {
       // (Unique, Any) -> Boolean  and
       // (Any, Unique) -> Boolean
       CheckUpperIs(node, Type::Boolean());
       break;
     }
     case IrOpcode::kObjectIsSmi:
       CheckValueInputIs(node, 0, Type::Any());
       CheckUpperIs(node, Type::Boolean());
       break;
@@ skipping 460 matching lines @@
     // Check inputs for all nodes in the block.
     for (size_t i = 0; i < block->NodeCount(); i++) {
       Node* node = block->NodeAt(i);
       CheckInputsDominate(schedule, block, node, static_cast<int>(i) - 1);
     }
   }
 }
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8