Add test for KT-19128

It looks like the problem was caused by incorrect merging of
StrictBasicValues and was fixed by commit
904c7f9c64.

 #KT-19128 Fixed

compiler/testData/codegen/box/checkcastOptimization/kt19128.kt

@@ -0,0 +1,26 @@
+class A
+
+class B
+
+class C
+
+fun foo(parameters: Any?): Any? {
+    var payload: Any? = null
+
+    if (parameters != null) {
+        if (parameters is A || parameters is B) {
+            payload = parameters
+        } else {
+            payload = "O"
+        }
+    }
+
+    if (payload is String) {
+        payload += "K"
+    }
+
+    return payload
+}
+
+fun box(): String =
+        "${foo(C())}"