[turbofan] Lower StringCharCodeAt to a dedicated builtin.

src/compiler/simplified-lowering.cc

Index: src/compiler/simplified-lowering.cc
diff --git a/src/compiler/simplified-lowering.cc b/src/compiler/simplified-lowering.cc
index 0fef9a6ad5fc907437e400f8fbe96bbf9cf69d07..f38401b09c923175c9c7b45e910f19224e195da4 100644
--- a/src/compiler/simplified-lowering.cc
+++ b/src/compiler/simplified-lowering.cc
@@ -2202,8 +2202,9 @@ class RepresentationSelector {
         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: {