[turbofan] Introduce a dedicated StringCharAt operator.

src/compiler/js-builtin-reducer.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/js-builtin-reducer.h"
 
 #include "src/compilation-dependencies.h"
 #include "src/compiler/access-builder.h"
 #include "src/compiler/js-graph.h"
 #include "src/compiler/node-matchers.h"
@@ skipping 1517 matching lines @@
         Node* receiver_length = effect = graph()->NewNode(
             simplified()->LoadField(AccessBuilder::ForStringLength()), receiver,
             effect, control);
 
         // Check if {index} is less than {receiver} length.
         Node* check = graph()->NewNode(simplified()->NumberLessThan(), index,
                                        receiver_length);
         Node* branch = graph()->NewNode(common()->Branch(BranchHint::kTrue),
                                         check, control);
 
+        // Return the character from the {receiver} as single character string.
         Node* if_true = graph()->NewNode(common()->IfTrue(), branch);
-        Node* vtrue;
-        {
-          // Load the character from the {receiver}.
-          vtrue = graph()->NewNode(simplified()->StringCharCodeAt(), receiver,
-                                   index, if_true);
-
-          // Return it as single character string.
-          vtrue = graph()->NewNode(simplified()->StringFromCharCode(), vtrue);
-        }
+        Node* vtrue = graph()->NewNode(simplified()->StringCharAt(), receiver,
+                                       index, if_true);
 
         // Return the empty string otherwise.
         Node* if_false = graph()->NewNode(common()->IfFalse(), branch);
         Node* vfalse = jsgraph()->EmptyStringConstant();
 
         control = graph()->NewNode(common()->Merge(2), if_true, if_false);
         Node* value =
             graph()->NewNode(common()->Phi(MachineRepresentation::kTagged, 2),
                              vtrue, vfalse, control);
 
@@ skipping 485 matching lines @@
   return jsgraph()->simplified();
 }
 
 JSOperatorBuilder* JSBuiltinReducer::javascript() const {
   return jsgraph()->javascript();
 }
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8