[turbofan] Introduce a dedicated StringCharAt operator.

src/builtins/builtins-string.cc

 // Copyright 2016 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/builtins/builtins-utils.h"
 #include "src/builtins/builtins.h"
 #include "src/code-factory.h"
 #include "src/code-stub-assembler.h"
 #include "src/regexp/regexp-utils.h"
 
@@ skipping 445 matching lines @@
 }
 
 // static
 void Builtins::Generate_StringGreaterThanOrEqual(
     compiler::CodeAssemblerState* state) {
   CodeStubAssembler assembler(state);
   GenerateStringRelationalComparison(
       &assembler, RelationalComparisonMode::kGreaterThanOrEqual);
 }
 
+// static
+void Builtins::Generate_StringCharAt(compiler::CodeAssemblerState* state) {
+  typedef compiler::Node Node;
+  CodeStubAssembler assembler(state);
+
+  Node* receiver = assembler.Parameter(0);
+  Node* position = assembler.Parameter(1);
+
+  // Load the character code at the {position} from the {receiver}.
+  Node* code = assembler.StringCharCodeAt(receiver, position,
+                                          CodeStubAssembler::INTPTR_PARAMETERS);
+
+  // And return the single character string with only that {code}
+  Node* result = assembler.StringFromCharCode(code);
+  assembler.Return(result);
+}
+
 // -----------------------------------------------------------------------------
 // ES6 section 21.1 String Objects
 
 // ES6 section 21.1.2.1 String.fromCharCode ( ...codeUnits )
 void Builtins::Generate_StringFromCharCode(
     compiler::CodeAssemblerState* state) {
   typedef CodeStubAssembler::Label Label;
   typedef compiler::Node Node;
   typedef CodeStubAssembler::Variable Variable;
   CodeStubAssembler assembler(state);
@@ skipping 1138 matching lines @@
         Runtime::kThrowIncompatibleMethodReceiver, context,
         assembler.HeapConstant(assembler.factory()->NewStringFromAsciiChecked(
             "String Iterator.prototype.next", TENURED)),
         iterator);
     assembler.Return(result);  // Never reached.
   }
 }
 
 }  // namespace internal
 }  // namespace v8