[IR] minor: make StageController an open class

This should make creating a new "do nothing" controller more straightforward.

compiler/ir/backend.js/src/org/jetbrains/kotlin/ir/backend/js/MutableController.kt

@@ -13,7 +13,7 @@ import org.jetbrains.kotlin.ir.declarations.impl.IrDeclarationBase
 import org.jetbrains.kotlin.ir.expressions.IrBody
 import org.jetbrains.kotlin.ir.util.isLocal
 
-open class MutableController(val context: JsIrBackendContext, val lowerings: List<Lowering>) : StageController {
+open class MutableController(val context: JsIrBackendContext, val lowerings: List<Lowering>) : StageController() {
 
     override var currentStage: Int = 0
 

compiler/ir/backend.js/src/org/jetbrains/kotlin/ir/backend/js/compiler.kt

@@ -49,7 +49,7 @@ fun compile(
     multiModule: Boolean = false,
     relativeRequirePath: Boolean = false
 ): CompilerResult {
-    stageController = object : StageController {}
+    stageController = StageController()
 
     val (moduleFragment: IrModuleFragment, dependencyModules, irBuiltIns, symbolTable, deserializer) =
         loadIr(project, mainModule, analyzer, configuration, allDependencies, friendDependencies)
@@ -86,9 +86,7 @@ fun compile(
         eliminateDeadDeclarations(allModules, context)
 
         // TODO investigate whether this is needed anymore
-        stageController = object : StageController {
-            override val currentStage: Int = controller.currentStage
-        }
+        stageController = StageController(controller.currentStage)
 
         val transformer = IrModuleToJsTransformer(
             context,