[turbofan] Introduce a dedicated StringCharAt operator.

src/interface-descriptors.cc

Index: src/interface-descriptors.cc
diff --git a/src/interface-descriptors.cc b/src/interface-descriptors.cc
index 2d51ae3acff1f4c2c21d8555053ae8d1305d249b..49e58f619e8fc834a4f4eb061140e32b050b7852 100644
--- a/src/interface-descriptors.cc
+++ b/src/interface-descriptors.cc
@@ -184,6 +184,20 @@ void StoreNamedTransitionDescriptor::InitializePlatformSpecific(
   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()};