[FIR] Check is for impossibility

^KT-58203 Fixed
^KT-62646

compiler/fir/analysis-tests/testData/resolveWithStdlib/contracts/fromSource/good/returnsImplies/safeCall.kt

@@ -13,7 +13,7 @@ fun test1(x: String?): Int? {
 @OptIn(ExperimentalContracts::class)
 fun test2(x: String?): Int? {
     <!WRONG_IMPLIES_CONDITION!>contract {
-        returnsNotNull() implies (x is Boolean)
+        returnsNotNull() implies (<!USELESS_IS_CHECK!>x is Boolean<!>)
     }<!>
 
     return x?.length