[turbofan] Improve typing for CheckBounds.

src/compiler/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 "src/compiler/simplified-lowering.h"
 
 #include <limits>
 
 #include "src/address-map.h"
 #include "src/base/bits.h"
@@ skipping 2086 matching lines @@
                    MachineRepresentation::kWord32);
         return;
       }
       case IrOpcode::kStringFromCharCode: {
         VisitUnop(node, UseInfo::TruncatingWord32(),
                   MachineRepresentation::kTagged);
         return;
       }
 
       case IrOpcode::kCheckBounds: {
-        if (TypeOf(node->InputAt(0))->Is(Type::Unsigned32())) {
+        Type* index_type = TypeOf(node->InputAt(0));
+        if (index_type->Is(Type::Unsigned32())) {
           VisitBinop(node, UseInfo::TruncatingWord32(),
                      MachineRepresentation::kWord32);
         } else {
           VisitBinop(node, UseInfo::CheckedSigned32AsWord32(),
                      UseInfo::TruncatingWord32(),
                      MachineRepresentation::kWord32);
         }
+        if (lower()) {
+          // The bounds check is redundant if we already know that
+          // the index is within the bounds of [0.0, length[.
+          if (index_type->Is(NodeProperties::GetType(node))) {
+            DeferReplacement(node, node->InputAt(0));
+          }
+        }
         return;
       }
       case IrOpcode::kCheckIf: {
         ProcessInput(node, 0, UseInfo::Bool());
         ProcessRemainingInputs(node, 1);
         SetOutput(node, MachineRepresentation::kNone);
         return;
       }
       case IrOpcode::kCheckNumber: {
         if (InputIs(node, Type::Number())) {
@@ skipping 1211 matching lines @@
         isolate(), graph()->zone(), callable.descriptor(), 0, flags,
         Operator::kNoProperties);
     to_number_operator_.set(common()->Call(desc));
   }
   return to_number_operator_.get();
 }
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8