[BE] Report if there are CR labels during InlineCodegen

compiler/backend/src/org/jetbrains/kotlin/codegen/inline/PsiSourceCompilerForInline.kt

@@ -437,9 +437,10 @@ fun getDeclarationLabels(lambdaOrFun: PsiElement?, descriptor: DeclarationDescri
     val result = HashSet<String>()
 
     if (lambdaOrFun != null) {
-        val label = LabelResolver.getLabelNamesIfAny(lambdaOrFun, addClassNameLabels = false)
-        if (label.isNotEmpty()) {
-            result.add(label.single().asString())
+        val labels = LabelResolver.getLabelNamesIfAny(lambdaOrFun, addClassNameLabels = false)
+        if (labels.isNotEmpty()) {
+            val label = labels.singleOrNull() ?: error("Multiple labels for $lambdaOrFun")
+            result.add(label.asString())
         }
     }