[FIR] Fix computing labels of anonymous functions (not lambdas)

compiler/testData/codegen/box/functions/localReturnInsideFunctionExpression.kt

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND_FIR: JVM_IR
 fun simple() = fun (): Boolean { return true }
 
 fun withLabel() = l@ fun (): Boolean { return@l true }
@@ -8,4 +7,4 @@ fun box(): String {
     if (!withLabel()()) return "Test withLabel failed"
 
     return "OK"
-}
+}