[turbofan] Eliminate redundant CheckString based on types.

src/compiler/js-typed-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 "src/code-factory.h"
 #include "src/compilation-dependencies.h"
 #include "src/compiler/access-builder.h"
 #include "src/compiler/js-graph.h"
 #include "src/compiler/js-typed-lowering.h"
 #include "src/compiler/linkage.h"
@@ skipping 1871 matching lines @@
   }
   if (vtrue_type->Is(false_type_) && vfalse_type->Is(true_type_)) {
     // Select(condition, vtrue:false, vfalse:true) => BooleanNot(condition)
     node->TrimInputCount(1);
     NodeProperties::ChangeOp(node, simplified()->BooleanNot());
     return Changed(node);
   }
   return NoChange();
 }
 
+Reduction JSTypedLowering::ReduceCheckString(Node* node) {
+  // TODO(bmeurer): Find a better home for this thing!
+  Node* const input = NodeProperties::GetValueInput(node, 0);
+  Type* const input_type = NodeProperties::GetType(input);
+  if (input_type->Is(Type::String())) {
+    ReplaceWithValue(node, input);
+    return Replace(input);
+  }
+  return NoChange();
+}
+
 Reduction JSTypedLowering::ReduceNumberRoundop(Node* node) {
   // TODO(bmeurer): Find a better home for this thing!
   Node* const input = NodeProperties::GetValueInput(node, 0);
   Type* const input_type = NodeProperties::GetType(input);
   if (input_type->Is(type_cache_.kIntegerOrMinusZeroOrNaN)) {
     return Replace(input);
   }
   return NoChange();
 }
 
@@ skipping 110 matching lines @@
     case IrOpcode::kJSForInStep:
       return ReduceJSForInStep(node);
     case IrOpcode::kJSGeneratorStore:
       return ReduceJSGeneratorStore(node);
     case IrOpcode::kJSGeneratorRestoreContinuation:
       return ReduceJSGeneratorRestoreContinuation(node);
     case IrOpcode::kJSGeneratorRestoreRegister:
       return ReduceJSGeneratorRestoreRegister(node);
     case IrOpcode::kSelect:
       return ReduceSelect(node);
+    case IrOpcode::kCheckString:
+      return ReduceCheckString(node);
     case IrOpcode::kNumberCeil:
     case IrOpcode::kNumberFloor:
     case IrOpcode::kNumberRound:
     case IrOpcode::kNumberTrunc:
       return ReduceNumberRoundop(node);
     default:
       break;
   }
   return NoChange();
 }
@@ skipping 34 matching lines @@
 }
 
 
 CompilationDependencies* JSTypedLowering::dependencies() const {
   return dependencies_;
 }
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8