JS: don't translate RHS of overloaded assignment operators twice. It can break compilation in some cases, for example, when there are lambdas in RHS.

Fix #KT-12808 #KT-12807

js/js.tests/test/org/jetbrains/kotlin/js/test/semantics/OperatorOverloadingTest.java

@@ -120,4 +120,8 @@ public final class OperatorOverloadingTest extends SingleFileTranslationTest {
     public void testOverloadUnaryOperationsViaExtensionFunctions() throws Exception {
         fooBoxTest();
     }
+
+    public void testLambdaRhs() throws Exception {
+        checkFooBoxIsOk();
+    }
 }