[turbofan] Lower StringCharCodeAt to a dedicated builtin.

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 2184 matching lines @@
       case IrOpcode::kStringLessThanOrEqual: {
         return VisitBinop(node, UseInfo::AnyTagged(),
                           MachineRepresentation::kTaggedPointer);
       }
       case IrOpcode::kStringCharAt: {
         VisitBinop(node, UseInfo::AnyTagged(), UseInfo::TruncatingWord32(),
                    MachineRepresentation::kTaggedPointer);
         return;
       }
       case IrOpcode::kStringCharCodeAt: {
+        // TODO(turbofan): Allow builtins to return untagged values.
         VisitBinop(node, UseInfo::AnyTagged(), UseInfo::TruncatingWord32(),
-                   MachineRepresentation::kWord32);
+                   MachineRepresentation::kTaggedSigned);
         return;
       }
       case IrOpcode::kStringFromCharCode: {
         VisitUnop(node, UseInfo::TruncatingWord32(),
                   MachineRepresentation::kTaggedPointer);
         return;
       }
       case IrOpcode::kStringFromCodePoint: {
         VisitUnop(node, UseInfo::TruncatingWord32(),
                   MachineRepresentation::kTaggedPointer);
@@ skipping 1240 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