[turbofan] Improve typing for string::length property.

src/compiler/access-builder.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/access-builder.h"
 #include "src/types-inl.h"
 
 namespace v8 {
 namespace internal {
 namespace compiler {
@@ skipping 43 matching lines @@
 // static
 FieldAccess AccessBuilder::ForMapInstanceType() {
   return {kTaggedBase, Map::kInstanceTypeOffset, Handle<Name>(),
           Type::UntaggedUnsigned8(), kMachUint8};
 }
 
 
 // static
 FieldAccess AccessBuilder::ForStringLength() {
   return {kTaggedBase, String::kLengthOffset, Handle<Name>(),
-          Type::SignedSmall(), kMachAnyTagged};
+          Type::Intersect(Type::UnsignedSmall(), Type::TaggedSigned()),
+          kMachAnyTagged};
 }
 
 
 // static
 FieldAccess AccessBuilder::ForValue() {
   return {kTaggedBase, JSValue::kValueOffset, Handle<Name>(), Type::Any(),
           kMachAnyTagged};
 }
 
 
@@ skipping 36 matching lines @@
     case kExternalFloat64Array:
       return {taggedness, header_size, Type::Number(), kMachFloat64};
   }
   UNREACHABLE();
   return {kUntaggedBase, 0, Type::None(), kMachNone};
 }
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8