[turbofan] Mark Typer::Visitor::TypeStringIndexOf unreachable

The type of StringIndexOf nodes is never recomputed since the operation
is simply changed on the original node.

Bug: chromium:769923
Change-Id: I3a2956ea69d43a56d22aff0607ac9869cf65533c
Reviewed-on: https://chromium-review.googlesource.com/718758Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48558}

src/compiler/typer.cc

@@ -1858,9 +1858,7 @@ 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::TypeStringIndexOf(Node* node) { UNREACHABLE(); }
 
 Type* Typer::Visitor::TypeCheckBounds(Node* node) {
   Type* index = Operand(node, 0);