IR, testing framework: if no tree labels were provided in source file, use default golden file.

compiler/ir/ir.tests/tests/org/jetbrains/kotlin/ir/AbstractIrTextTestCase.kt

@@ -90,6 +90,10 @@ abstract class AbstractIrTextTestCase : AbstractIrGeneratorTestCase() {
                 }
             }
 
+            if (treeFiles.isEmpty()) {
+                treeFiles.add(IrTreeFileLabel(File(dir, testFile.name.replace(".kt", ".txt")), 0))
+            }
+
             return Expectations(regexps, treeFiles)
         }