[turbofan] Introduce dedicated StringIndexOf operator.

src/compiler/verifier.cc

Index: src/compiler/verifier.cc
diff --git a/src/compiler/verifier.cc b/src/compiler/verifier.cc
index f554dd7961f079927b5661ba952ee15bb9688cfb..80f613d07399037a23b30479799c8b502a696a6c 100644
--- a/src/compiler/verifier.cc
+++ b/src/compiler/verifier.cc
@@ -926,12 +926,20 @@ void Verifier::Visitor::Check(Node* node) {
       CheckValueInputIs(node, 0, Type::Number());
       CheckTypeIs(node, Type::String());
       break;
-    case IrOpcode::kReferenceEqual: {
+    case IrOpcode::kStringIndexOf:
+      // (String, String, SignedSmall) -> SignedSmall
+      CheckValueInputIs(node, 0, Type::String());
+      CheckValueInputIs(node, 1, Type::String());
+      CheckValueInputIs(node, 2, Type::SignedSmall());
+      CheckTypeIs(node, Type::SignedSmall());
+      break;
+
+    case IrOpcode::kReferenceEqual:
       // (Unique, Any) -> Boolean  and
       // (Any, Unique) -> Boolean
       CheckTypeIs(node, Type::Boolean());
       break;
-    }
+
     case IrOpcode::kObjectIsCallable:
     case IrOpcode::kObjectIsNonCallable:
     case IrOpcode::kObjectIsNumber: