CAST_NEVER_SUCCEEDS: do not report when casting nullable to nullable #KT-260 Fixed

compiler/testData/diagnostics/tests/cast/NullableToNullable.kt

@@ -0,0 +1,7 @@
+// From KT-13324: always succeeds
+val x = null <!USELESS_CAST!>as String?<!>
+// From KT-260: sometimes succeeds
+fun foo(a: String?): Int? {
+    val c = a as? Int?
+    return c
+}