FIR checker: warn useless as and is

compiler/tests-spec/testData/diagnostics/notLinked/contracts/declarations/contractBuilder/effects/returns/neg/7.fir.kt

@@ -6,7 +6,7 @@ import kotlin.contracts.*
 // TESTCASE NUMBER: 1
 fun <T : Number?> T.case_1() {
     contract { returns() implies (this@case_1 is T) }
-    if (!(this@case_1 is T)) throw Exception()
+    if (!(<!USELESS_IS_CHECK!>this@case_1 is T<!>)) throw Exception()
 }
 
 // TESTCASE NUMBER: 2