[turbofan] Introduce a dedicated StringCharAt operator.

src/interface-descriptors.h

 // 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.
 
 #ifndef V8_CALL_INTERFACE_DESCRIPTOR_H_
 #define V8_CALL_INTERFACE_DESCRIPTOR_H_
 
 #include <memory>
 
 #include "src/assembler.h"
@@ skipping 55 matching lines @@
   V(Builtin)                              \
   V(ArrayNoArgumentConstructor)           \
   V(ArraySingleArgumentConstructor)       \
   V(ArrayNArgumentsConstructor)           \
   V(Compare)                              \
   V(BinaryOp)                             \
   V(BinaryOpWithAllocationSite)           \
   V(BinaryOpWithVector)                   \
   V(CountOp)                              \
   V(StringAdd)                            \
+  V(StringCharAt)                         \
   V(StringCompare)                        \
   V(SubString)                            \
   V(Keyed)                                \
   V(Named)                                \
   V(HasProperty)                          \
   V(ForInFilter)                          \
   V(GetProperty)                          \
   V(CallHandler)                          \
   V(ArgumentAdaptor)                      \
   V(ApiCallback)                          \
@@ skipping 596 matching lines @@
  public:
   DECLARE_DESCRIPTOR(CountOpDescriptor, CallInterfaceDescriptor)
 };
 
 class StringAddDescriptor : public CallInterfaceDescriptor {
  public:
   DEFINE_PARAMETERS(kLeft, kRight)
   DECLARE_DESCRIPTOR(StringAddDescriptor, CallInterfaceDescriptor)
 };
 
+class StringCharAtDescriptor final : public CallInterfaceDescriptor {
+ public:
+  DEFINE_PARAMETERS(kReceiver, kPosition)
+  DECLARE_DESCRIPTOR_WITH_CUSTOM_FUNCTION_TYPE(StringCharAtDescriptor,
+                                               CallInterfaceDescriptor)
+};
 
 class StringCompareDescriptor : public CallInterfaceDescriptor {
  public:
   DEFINE_PARAMETERS(kLeft, kRight)
   DECLARE_DESCRIPTOR(StringCompareDescriptor, CallInterfaceDescriptor)
 
   static const Register LeftRegister();
   static const Register RightRegister();
 };
 
@@ skipping 158 matching lines @@
 }  // namespace v8
 
 
 #if V8_TARGET_ARCH_ARM64
 #include "src/arm64/interface-descriptors-arm64.h"
 #elif V8_TARGET_ARCH_ARM
 #include "src/arm/interface-descriptors-arm.h"
 #endif
 
 #endif  // V8_CALL_INTERFACE_DESCRIPTOR_H_