[FIR] Properly process `!is` branches in exhaustiveness checker

In the scope of KT-80461 the new simplified mechanism for checking
exhaustiveness with sealed subject was introduced. For `!is Type` it
considered all the sealed subclasses which are not subtype of `Type` to
be checked. But it's actually incorrect in case if the `Type` is an
inheritor of the sealed subclass itself, which caused false-negative
`NO_ELSE_IN_WHEN` happen.

^KT-82841 Fixed

compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/transformers/FirWhenExhaustivenessComputer.kt

diff --git a/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/transformers/FirWhenExhaustivenessComputer.kt b/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/transformers/FirWhenExhaustivenessComputer.kt
index a249c22..00288df 100644
--- a/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/transformers/FirWhenExhaustivenessComputer.kt
+++ b/compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/transformers/FirWhenExhaustivenessComputer.kt
@@ -511,9 +511,9 @@
         }
 
         private fun processBranchUsingSubtyping(symbolToCheck: FirClassSymbol<*>, isNegated: Boolean, data: Info) {
-            val predicate: (FirClassSymbol<*>) -> Boolean = {
-                it.isSubclassOf(
-                    symbolToCheck.toLookupTag(),
+            fun FirClassSymbol<*>.isSubclassOf(other: FirClassSymbol<*>): Boolean {
+                return this.isSubclassOf(
+                    other.toLookupTag(),
                     data.session,
                     isStrict = false,
                     lookupInterfaces = true
@@ -522,10 +522,14 @@
 
             val subclassesCheckedByTheBranch = when (isNegated) {
                 // `<subj> is Type`, `<subj> == Type` branches
-                false -> data.allSubclasses.filter(predicate)
+                false -> data.allSubclasses.filter { subclass ->
+                    subclass.isSubclassOf(symbolToCheck)
+                }
 
                 // `<subj> !is Type`, `<subj> != Type` branches
-                true -> data.allSubclasses.filterNot(predicate)
+                true -> data.allSubclasses.filter { subclass ->
+                    !subclass.isSubclassOf(symbolToCheck) && !symbolToCheck.isSubclassOf(subclass)
+                }
             }
 
             // subclassesCheckedByTheBranch

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

diff --git a/compiler/testData/diagnostics/tests/sealed/nonExhaustiveByNegation.fir.kt b/compiler/testData/diagnostics/tests/sealed/nonExhaustiveByNegation.fir.kt
deleted file mode 100644
index 279badd..0000000
--- a/compiler/testData/diagnostics/tests/sealed/nonExhaustiveByNegation.fir.kt
+++ /dev/null
@@ -1,17 +0,0 @@
-// RUN_PIPELINE_TILL: BACKEND
-// ISSUE: KT-82841
-
-sealed class Base
-
-abstract class Derived : Base()
-
-class Implementation : Derived()
-
-fun test_1(x: Base) {
-    val value = when (x) {
-        !is Implementation -> 1
-    }
-}
-
-/* GENERATED_FIR_TAGS: classDeclaration, functionDeclaration, integerLiteral, isExpression, localProperty,
-propertyDeclaration, sealed, whenExpression, whenWithSubject */

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

diff --git a/compiler/testData/diagnostics/tests/sealed/nonExhaustiveByNegation.kt b/compiler/testData/diagnostics/tests/sealed/nonExhaustiveByNegation.kt
index 4bf1d32..2fe9b60 100644
--- a/compiler/testData/diagnostics/tests/sealed/nonExhaustiveByNegation.kt
+++ b/compiler/testData/diagnostics/tests/sealed/nonExhaustiveByNegation.kt
@@ -1,4 +1,5 @@
-// RUN_PIPELINE_TILL: BACKEND
+// RUN_PIPELINE_TILL: FRONTEND
+// FIR_IDENTICAL
 // ISSUE: KT-82841
 
 sealed class Base