[turbofan] Introduce dedicated StringIndexOf operator.

src/builtins/builtins-string.cc

Index: src/builtins/builtins-string.cc
diff --git a/src/builtins/builtins-string.cc b/src/builtins/builtins-string.cc
index f2b784cf053b409f5a7982329d3839582afa9d27..bbe246af0057b09e142e5f10f7c768567be80db0 100644
--- a/src/builtins/builtins-string.cc
+++ b/src/builtins/builtins-string.cc
@@ -864,8 +864,6 @@ TF_BUILTIN(StringIndexOf, StringBuiltinsAssembler) {
   Node* search_string = Parameter(1);
   Node* position = Parameter(2);
 
-  Label call_runtime(this);
-
   Node* instance_type = LoadInstanceType(receiver);
   Node* search_string_instance_type = LoadInstanceType(search_string);