[turbofan] Introduce a dedicated StringCharAt operator.

src/interface-descriptors.cc

 // Copyright 2012 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/interface-descriptors.h"
 
 namespace v8 {
 namespace internal {
 
 
@@ skipping 166 matching lines @@
     CallInterfaceDescriptorData* data) {
   Register registers[] = {
       ReceiverRegister(), FieldOffsetRegister(), MapRegister(),
       ValueRegister(),    SlotRegister(),        VectorRegister(),
       NameRegister(),
   };
   int len = arraysize(registers) - kStackArgumentsCount;
   data->InitializePlatformSpecific(len, registers);
 }
 
+void StringCharAtDescriptor::InitializePlatformIndependent(
+    CallInterfaceDescriptorData* data) {
+  // kReceiver, kPosition
+  MachineType machine_types[] = {MachineType::AnyTagged(),
+                                 MachineType::IntPtr()};
+  data->InitializePlatformIndependent(arraysize(machine_types), 0,
+                                      machine_types);
+}
+
+void StringCharAtDescriptor::InitializePlatformSpecific(
+    CallInterfaceDescriptorData* data) {
+  DefaultInitializePlatformSpecific(data, kParameterCount);
+}
+
 void StringCompareDescriptor::InitializePlatformSpecific(
     CallInterfaceDescriptorData* data) {
   Register registers[] = {LeftRegister(), RightRegister()};
   data->InitializePlatformSpecific(arraysize(registers), registers);
 }
 
 void TypeConversionDescriptor::InitializePlatformSpecific(
     CallInterfaceDescriptorData* data) {
   Register registers[] = {ArgumentRegister()};
   data->InitializePlatformSpecific(arraysize(registers), registers);
@@ skipping 325 matching lines @@
     CallInterfaceDescriptorData* data) {
   // kNumberOfArguments, kFirstArgument, kFunctionEntry
   MachineType machine_types[] = {MachineType::Int32(), MachineType::Pointer(),
                                  MachineType::Pointer()};
   data->InitializePlatformIndependent(arraysize(machine_types), 0,
                                       machine_types);
 }
 
 }  // namespace internal
 }  // namespace v8