Make nullable check less verbose in JS-backend.

For check what variable is equal null or undefined enough to compare with null (or undefined).
See ECMAScript specification 11.9.1, 11.9.2, 11.9.3

js/js.translator/testFiles/expression/equals/cases/equalsNullOrUndefined.kt

@@ -0,0 +1,18 @@
+package foo
+
+fun box() : String {
+    val a: Int? = null
+    val r = a == null
+    if (!r || a != null)
+      return "wrong result on simple nullable check"
+
+    var i = 0;
+    fun foo(): Int? = ++i;
+    if (foo() == null)
+      return "wrong result on nullable check with side effects"
+
+    if (i != 1)
+      return "wrong affects when using nullable check with side effects"
+
+    return "OK"
+}