[turbofan] Introduce a dedicated StringCharAt operator.

src/interface-descriptors.h

Index: src/interface-descriptors.h
diff --git a/src/interface-descriptors.h b/src/interface-descriptors.h
index 485ed7930268f1341e5b9c10c8f28c6e92f1aa10..7a929269e1c611603dbe2a08480f273b2cc30176 100644
--- a/src/interface-descriptors.h
+++ b/src/interface-descriptors.h
@@ -73,6 +73,7 @@ class PlatformInterfaceDescriptor;
   V(BinaryOpWithVector)                   \
   V(CountOp)                              \
   V(StringAdd)                            \
+  V(StringCharAt)                         \
   V(StringCompare)                        \
   V(SubString)                            \
   V(Keyed)                                \
@@ -689,6 +690,12 @@ class StringAddDescriptor : public CallInterfaceDescriptor {
   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: