FIR checker: warn useless as and is

compiler/testData/diagnostics/tests/smartCasts/safeAs.fir.kt

@@ -1,13 +1,13 @@
 // See also KT-10992: we should have no errors for all unsafe hashCode() calls
 
 fun foo(arg: Any?) {
-    val x = arg as? Any ?: return
+    val x = arg <!USELESS_CAST!>as? Any<!> ?: return
     arg.hashCode()
     x.hashCode()
 }
 
 fun bar(arg: Any?) {
-    arg as? Any ?: return
+    arg <!USELESS_CAST!>as? Any<!> ?: return
     arg.hashCode()
 }