[IR] Correct work of the new MFVC lowering classes

#KT-1179

compiler/testData/codegen/box/valueClasses/classFlattening.kt

@@ -2,6 +2,7 @@
 // TARGET_BACKEND: JVM_IR
 // WORKS_WHEN_VALUE_CLASS
 // LANGUAGE: +ValueClasses
+// CHECK_BYTECODE_LISTING
 
 @JvmInline
 value class IC(val x: UInt)
@@ -18,4 +19,12 @@ value class GreaterMFVC(val x: SimpleMFVC, val y: IC, val z: SimpleMFVC)
 
 fun gmfvc(ic: IC, x: GreaterMFVC, ic1: UInt) = smfvc(ic, x.x, 0U) + ic(x.y) + smfvc(IC(0U), x.z, ic1)
 
-fun box() = "OK" // todo real test
+fun box(): String {
+    val o1 = IC(2U)
+    require(ic(o1) == 2U)
+    val o2 = SimpleMFVC(1U, o1, "3")
+    require(smfvc(IC(4U), o2, 5U) == 12U)
+    val o3 = GreaterMFVC(o2, IC(6U), SimpleMFVC(7U, IC(8U), "9"))
+    require(gmfvc(IC(10U), o3, 11U) == 45U)
+    return "OK"
+}