FIR checker: warn useless as and is

compiler/testData/diagnostics/tests/cast/AsInPropertyAndPropertyAccessor.fir.kt

@@ -4,7 +4,7 @@ fun test() {
     val a = 1 as Any?
     val b: Number = 1 as Number
     val c = null as String?
-    val d: Number = 1 as Int
+    val d: Number = 1 <!USELESS_CAST!>as Int<!>
 }
 
 val c1 get() = 1 as Number
@@ -14,4 +14,4 @@ val d: Number
     get() {
         1 as Number
         return 1 as Number
-    }
+    }