[FIR] Check is for impossibility

^KT-58203 Fixed
^KT-62646

compiler/testData/diagnostics/tests/inference/coercionToUnit/coercionToUnitWithNothingType.fir.kt

@@ -45,7 +45,7 @@ fun test(i: Inv<Nothing>, iUnit: Inv<Unit>) {
         run<dynamic> { "" }
     }
 
-    if (iUnit is String) {
+    if (<!USELESS_IS_CHECK!>iUnit is String<!>) {
         launch {
             run(A.<!INFERRED_TYPE_VARIABLE_INTO_EMPTY_INTERSECTION_WARNING!>flexible<!>(iUnit)) { 42 }
         }