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.translator/testData/operatorOverloading/cases/lambdaRhs.kt

@@ -0,0 +1,27 @@
+package foo
+
+var global = ""
+
+fun log(message: String) {
+    global += message
+}
+
+fun test1(): String {
+    val list = mutableListOf<(Int) -> Unit>()
+
+    var result = ""
+    list += { log("<$it>") }
+    list += { result += "($it)" }
+    list += { result += "[$it]"; result += "{$it}" }
+
+    for(f in list) f(1)
+
+    return result
+}
+
+fun box(): String {
+    assertEquals("(1)[1]{1}", test1())
+    assertEquals(global, "<1>")
+
+    return "OK"
+}