FIR checker: warn useless as and is

compiler/testData/diagnostics/tests/when/When.fir.kt

@@ -20,9 +20,9 @@ fun foo() : Int {
     val x = 1
     when (x) {
       is String -> 1
-      !is Int -> 1
-      is Any? -> 1
-      is Any -> 1
+      <!USELESS_IS_CHECK!>!is Int<!> -> 1
+      <!USELESS_IS_CHECK!>is Any?<!> -> 1
+      <!USELESS_IS_CHECK!>is Any<!> -> 1
       s -> 1
       1 -> 1
       1 <!OVERLOAD_RESOLUTION_AMBIGUITY!>+<!> <!UNRESOLVED_REFERENCE!>a<!> -> 1

compiler/testData/diagnostics/tests/when/WhenTypeDisjunctions.fir.kt

@@ -23,7 +23,7 @@ fun foo(s: Any): String {
 
 fun bar(s: Any): String {
     val x = when (s) {
-        is String -> s as String // meaningless
+        is String -> s <!USELESS_CAST!>as String<!> // meaningless
         is Int -> "$s"
         else -> return ""
     }