Detect redundant 'is' check

#KT-14187 Fixed

compiler/testData/diagnostics/tests/sealed/ExhaustiveOnRoot.kt

@@ -9,7 +9,7 @@ sealed class Expr : Stmt() {
 fun test(x: Stmt): String =
         when (x) {
             is Expr -> "expr"
-            is Stmt -> "stmt"
+            <!USELESS_IS_CHECK!>is Stmt<!> -> "stmt"
         }
 
 fun test2(x: Stmt): String =
@@ -19,5 +19,5 @@ fun test2(x: Stmt): String =
 
 fun test3(x: Expr): String =
         when (x) {
-            is Stmt -> "stmt"
+            <!USELESS_IS_CHECK!>is Stmt<!> -> "stmt"
         }

compiler/testData/diagnostics/tests/sealed/NonExhaustiveWhenWithAnyCase.kt

@@ -9,7 +9,7 @@ sealed class Sealed {
 fun foo(s: Sealed): Int {
     return <!NO_ELSE_IN_WHEN!>when<!>(s) {
         is Sealed.First -> 1
-        !is Any -> 0
+        <!USELESS_IS_CHECK!>!is Any<!> -> 0
     }
 }