[K/JS] Fix simple suspend functions compilation using ES2015 generators

^KT-79359 Fixed

Author: JSMonk

compiler/ir/backend.js/src/org/jetbrains/kotlin/ir/backend/js/lower/coroutines/JsSuspendFunctionWithGeneratorsLowering.kt

@@ -47,7 +47,6 @@ class JsSuspendFunctionWithGeneratorsLowering(private val context: JsIrBackendCo
     }
 
     private fun transformSuspendFunction(function: IrSimpleFunction): List<IrFunction>? {
-        val originalReturnType = function.returnType.also { function.returnType = context.irBuiltIns.anyNType }
         val body = function.body ?: return null
         return when (val functionKind = getSuspendFunctionKind(context, function, body, includeSuspendLambda = false)) {
             is SuspendFunctionKind.NO_SUSPEND_CALLS -> null
@@ -56,6 +55,8 @@ class JsSuspendFunctionWithGeneratorsLowering(private val context: JsIrBackendCo
                 null
             }
             is SuspendFunctionKind.NEEDS_STATE_MACHINE -> {
+                val originalReturnType = function.returnType
+                function.returnType = context.irBuiltIns.anyNType
                 generateGeneratorAndItsWrapper(function, body, originalReturnType)
             }
         }

compiler/testData/codegen/box/coroutines/implicitUnitReturn.kt

@@ -1,7 +1,6 @@
 // KT-79359
 // TARGET_BACKEND: JS_IR
 // TARGET_BACKEND: JS_IR_ES6
-// IGNORE_BACKEND: JS_IR_ES6
 // WITH_STDLIB
 // WITH_COROUTINES