[turbofan] Introduce CheckString simplified operator.

src/compiler/redundancy-elimination.cc

 // Copyright 2016 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/redundancy-elimination.h"
 
 #include "src/compiler/node-properties.h"
 
 namespace v8 {
 namespace internal {
 namespace compiler {
 
 RedundancyElimination::RedundancyElimination(Editor* editor, Zone* zone)
     : AdvancedReducer(editor), node_checks_(zone), zone_(zone) {}
 
 RedundancyElimination::~RedundancyElimination() {}
 
 Reduction RedundancyElimination::Reduce(Node* node) {
   switch (node->opcode()) {
     case IrOpcode::kCheckBounds:
     case IrOpcode::kCheckFloat64Hole:
     case IrOpcode::kCheckIf:
     case IrOpcode::kCheckNumber:
+    case IrOpcode::kCheckString:
     case IrOpcode::kCheckTaggedHole:
     case IrOpcode::kCheckTaggedPointer:
     case IrOpcode::kCheckTaggedSigned:
     case IrOpcode::kCheckedFloat64ToInt32:
     case IrOpcode::kCheckedInt32Add:
     case IrOpcode::kCheckedInt32Sub:
     case IrOpcode::kCheckedInt32Div:
     case IrOpcode::kCheckedInt32Mod:
     case IrOpcode::kCheckedInt32Mul:
     case IrOpcode::kCheckedTaggedToFloat64:
@@ skipping 194 matching lines @@
       node_checks_.Set(node, checks);
       return Changed(node);
     }
   }
   return NoChange();
 }
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8