JS: don't optimize when over external enum

See KT-21468

js/js.translator/testData/box/expression/when/externalEnumSubject.js

@@ -0,0 +1,4 @@
+var X = {
+    A: {},
+    B: {}
+};

js/js.translator/testData/box/expression/when/externalEnumSubject.kt

@@ -0,0 +1,16 @@
+// EXPECTED_REACHABLE_NODES: 1133
+external enum class X {
+    A, B
+}
+
+fun box(): String {
+    if (foo(X.A) != "!") return "fail1: ${foo(X.A)}"
+    if (foo(X.B) != "@") return "fail2: ${foo(X.B)}"
+
+    return "OK"
+}
+
+fun foo(x: X) = when (x) {
+    X.A -> "!"
+    X.B -> "@"
+}