[turbofan] Introduce a dedicated StringCharAt operator.

src/builtins/builtins.h

Index: src/builtins/builtins.h
diff --git a/src/builtins/builtins.h b/src/builtins/builtins.h
index 49508de07a37524e8c8dd2a83e77f6fba37a84b3..20d0ce43ce611e94e7b12e85a47b48261fd69d07 100644
--- a/src/builtins/builtins.h
+++ b/src/builtins/builtins.h
@@ -104,6 +104,7 @@ namespace internal {
   TFS(StringLessThanOrEqual, BUILTIN, kNoExtraICState, Compare)                \
   TFS(StringGreaterThan, BUILTIN, kNoExtraICState, Compare)                    \
   TFS(StringGreaterThanOrEqual, BUILTIN, kNoExtraICState, Compare)             \
+  TFS(StringCharAt, BUILTIN, kNoExtraICState, StringCharAt)                    \
                                                                                \
   /* Interpreter */                                                            \
   ASM(InterpreterEntryTrampoline)                                              \