[FIR] Check is for impossibility

^KT-58203 Fixed
^KT-62646

compiler/testData/diagnostics/tests/sealed/ExhaustiveWhenNegatedTwice.fir.kt

@@ -0,0 +1,17 @@
+sealed class Sealed(val x: Int) {
+    object First: Sealed(12)
+    open class NonFirst(x: Int, val y: Int): Sealed(x) {
+        object Second: NonFirst(34, 2)
+        object Third: NonFirst(56, 3)
+    }
+    object Last: Sealed(78)
+}
+
+fun foo(s: Sealed): Int {
+    return when(s) {
+        !is Sealed.First -> 1
+        <!USELESS_IS_CHECK!>!is Sealed.Last<!> -> 0
+        // no else required
+    }
+}
+

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

@@ -1,4 +1,3 @@
-// FIR_IDENTICAL
 sealed class Sealed(val x: Int) {
     object First: Sealed(12)
     open class NonFirst(x: Int, val y: Int): Sealed(x) {