JS backend: fix equality check for nullable types

* Generate simple check when compare with `null` literal.
* Use Kotlin.equals if type of receiver or argument is nullable.
* Don't call equals method if any of receiver or argument is null (at runtime, not only literal). For more information see KT-4356.

 #KT-7530 fixed
 #KT-7916 fixed

js/js.translator/testData/expression/equals/cases/compareNullablesWithCustomEquals.kt

@@ -0,0 +1,47 @@
+package foo
+
+class A {
+    override fun equals(other: Any?) = this === other
+}
+
+fun box(): String {
+    val a: A? = null
+    val b: A? = null
+    val c: A? = A()
+    val d: A? = A()
+    val e: A = A()
+
+    // compare nullable vals with null
+    testTrue { a == b }
+    testTrue { a == a }
+    testFalse { a != b }
+    testFalse { a != a }
+
+    // compare null and non-null inside nullable vals
+    testFalse { a == c }
+    testTrue { a != c }
+    testFalse { c == a }
+    testTrue { c != a }
+
+    // compare nullables vals with non-null
+    testFalse { c == d }
+    testTrue { c == c }
+    testTrue { c != d }
+    testFalse { d == c }
+    testTrue { d != c }
+    testFalse { d != d }
+
+    // compare nullable val with null with non-nullable
+    testFalse { a == e }
+    testTrue { a != e }
+    testFalse { e == a }
+    testTrue { e != a }
+
+    // compare nullable val with non-null with non-nullable
+    testFalse { c == e }
+    testTrue { c != e }
+    testFalse { e == c }
+    testTrue { e != c }
+
+    return "OK"
+}