[turbofan] Introduce dedicated StringIndexOf operator.

src/compiler/simplified-operator.h

Index: src/compiler/simplified-operator.h
diff --git a/src/compiler/simplified-operator.h b/src/compiler/simplified-operator.h
index 5481030272b01c5b517bba75f494dc8e11acaa09..4cc1c98959bb7123e98ec34a999e720f0478403d 100644
--- a/src/compiler/simplified-operator.h
+++ b/src/compiler/simplified-operator.h
@@ -356,6 +356,7 @@ class V8_EXPORT_PRIVATE SimplifiedOperatorBuilder final
   const Operator* StringCharCodeAt();
   const Operator* StringFromCharCode();
   const Operator* StringFromCodePoint(UnicodeEncoding encoding);
+  const Operator* StringIndexOf();
 
   const Operator* PlainPrimitiveToNumber();
   const Operator* PlainPrimitiveToWord32();