FIR checkers: report SMARTCAST_IMPOSSIBLE

compiler/testData/diagnostics/tests/smartCasts/inference/unneededUnstableSmartcast.kt

@@ -0,0 +1,8 @@
+// FIR_IDENTICAL
+class Test<E>(var c: Collection<E>) {
+    fun test() {
+        if (c is List<*>) {
+            c.size // smartcast is unstable, but `size` is also available from `Collection`, so there should not be any error
+        }
+    }
+}