JS: fix bug in temporary variable eliminator

The problem was in considering `a` as trivial in following case:

```
var a = b;
```

However, that's wrong assumption, since `b` can be temporary variable
itself which is further substituted by a non-trivial expression.

js/js.translator/testData/box/inlineEvaluationOrder/temporaryVarNonTrivial.kt

@@ -0,0 +1,31 @@
+// EXPECTED_REACHABLE_NODES: 1002
+var log = ""
+
+fun bar(): A {
+    log += "foo;"
+    return A()
+}
+
+class A {
+    fun f() {
+        log += "f;"
+    }
+
+    fun g() {
+        log += "g;"
+    }
+}
+
+inline fun <T> with(x: T, a: T.() -> Unit) = x.a()
+
+fun box(): String {
+    with(bar()) {
+        f()
+        g()
+    }
+
+    if (log != "foo;f;g;") return "fail: $log"
+
+    return "OK"
+}
+