[turbofan] Introduce a dedicated StringCharAt operator.

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 5c583ae90ffb65253f314b45690edceb12a9b48d..0fef9a6ad5fc907437e400f8fbe96bbf9cf69d07 100644
--- a/src/compiler/simplified-lowering.cc
+++ b/src/compiler/simplified-lowering.cc
@@ -2196,6 +2196,11 @@ class RepresentationSelector {
         return VisitBinop(node, UseInfo::AnyTagged(),
                           MachineRepresentation::kTaggedPointer);
       }
+      case IrOpcode::kStringCharAt: {
+        VisitBinop(node, UseInfo::AnyTagged(), UseInfo::TruncatingWord32(),
+                   MachineRepresentation::kTaggedPointer);
+        return;
+      }
       case IrOpcode::kStringCharCodeAt: {
         VisitBinop(node, UseInfo::AnyTagged(), UseInfo::TruncatingWord32(),
                    MachineRepresentation::kWord32);