[turbofan] Introduce a dedicated StringCharAt operator.

src/compiler/effect-control-linearizer.cc

Index: src/compiler/effect-control-linearizer.cc
diff --git a/src/compiler/effect-control-linearizer.cc b/src/compiler/effect-control-linearizer.cc
index 0a8fbd900d9ddafe7f95c1c1146961e6bfaa193d..6b72b14249e1f5a8d5c39b92a952509359230c08 100644
--- a/src/compiler/effect-control-linearizer.cc
+++ b/src/compiler/effect-control-linearizer.cc
@@ -745,6 +745,9 @@ bool EffectControlLinearizer::TryWireInStateEffect(Node* node,
     case IrOpcode::kStringFromCodePoint:
       state = LowerStringFromCodePoint(node, *effect, *control);
       break;
+    case IrOpcode::kStringCharAt:
+      state = LowerStringCharAt(node, *effect, *control);
+      break;
     case IrOpcode::kStringCharCodeAt:
       state = LowerStringCharCodeAt(node, *effect, *control);
       break;
@@ -2226,6 +2229,22 @@ EffectControlLinearizer::LowerArrayBufferWasNeutered(Node* node, Node* effect,
 }
 
 EffectControlLinearizer::ValueEffectControl
+EffectControlLinearizer::LowerStringCharAt(Node* node, Node* effect,
+                                           Node* control) {
+  Callable const callable = CodeFactory::StringCharAt(isolate());
+  Operator::Properties properties = Operator::kNoThrow | Operator::kNoWrite;
+  CallDescriptor::Flags flags = CallDescriptor::kNoFlags;
+  CallDescriptor* desc = Linkage::GetStubCallDescriptor(
+      isolate(), graph()->zone(), callable.descriptor(), 0, flags, properties);
+  node->InsertInput(graph()->zone(), 0,
+                    jsgraph()->HeapConstant(callable.code()));
+  node->InsertInput(graph()->zone(), 3, jsgraph()->NoContextConstant());
+  node->InsertInput(graph()->zone(), 4, effect);
+  NodeProperties::ChangeOp(node, common()->Call(desc));
+  return ValueEffectControl(node, node, control);
+}
+
+EffectControlLinearizer::ValueEffectControl
 EffectControlLinearizer::LowerStringCharCodeAt(Node* node, Node* effect,
                                                Node* control) {
   Node* subject = node->InputAt(0);