[JS IR BE] Fix enum entry initialization

Keep helper fields uninitialized. This way thier initialization
would not depend on field initialization order.

js/js.translator/testData/box/regression/enumEntryInitOrder.kt

@@ -0,0 +1,10 @@
+// EXPECTED_REACHABLE_NODES: 1309
+// KJS_WITH_FULL_RUNTIME
+
+enum class E { A, B }
+val x: Any = E.A
+
+fun box(): String {
+    if (x !== E.A) return "Fail"
+    return "OK"
+}