[turbofan] Introduce dedicated StringIndexOf operator.

src/compiler/typer.cc

Index: src/compiler/typer.cc
diff --git a/src/compiler/typer.cc b/src/compiler/typer.cc
index 7fa1ffdf27fdb443b02d763c61bc942e9f4fd543..4a3a9dd15323236cf86ca78ae3e96dd576161e09 100644
--- a/src/compiler/typer.cc
+++ b/src/compiler/typer.cc
@@ -1764,6 +1764,10 @@ Type* Typer::Visitor::TypeStringFromCodePoint(Node* node) {
   return TypeUnaryOp(node, StringFromCodePointTyper);
 }
 
+Type* Typer::Visitor::TypeStringIndexOf(Node* node) {
+  return Type::Range(-1.0, String::kMaxLength - 1.0, zone());
+}
+
 Type* Typer::Visitor::TypeCheckBounds(Node* node) {
   Type* index = Operand(node, 0);
   Type* length = Operand(node, 1);