JS/RTTI: avoid possible side-effect for is-check on nullable type

#KT-7038 fixed

js/js.translator/testData/rtti/cases/sideEffectMethod.kt

@@ -0,0 +1,21 @@
+package foo
+
+var counter = 0
+
+class A {
+    fun getCounter(): Any? =
+        when (counter++) {
+            0 -> "0"
+            1 -> null
+            else -> counter
+        }
+}
+
+fun box(): String {
+    assertEquals(false, A().getCounter() is Int?, "(1)")
+    assertEquals(true, A().getCounter() is Int?, "(2)")
+    assertEquals(true, A().getCounter() is Int?, "(3)")
+    assertEquals(3, counter)
+
+    return "OK"
+}

js/js.translator/testData/rtti/cases/sideEffectProperty.kt

@@ -0,0 +1,22 @@
+package foo
+
+var counter = 0
+
+class A {
+    val x: Any?
+        get() =
+            when (counter++) {
+                0 -> "0"
+                1 -> null
+                else -> counter
+            }
+}
+
+fun box(): String {
+    assertEquals(false, A().x is Int?, "(1)")
+    assertEquals(true, A().x is Int?, "(2)")
+    assertEquals(true, A().x is Int?, "(3)")
+    assertEquals(3, counter)
+
+    return "OK"
+}