[FIR] Utilize equality compatibility logic for cast checks

This makes it more consistent and fixes some
overlooked corner cases. Also it was decided
on the last equality applicability DM
(KT-62646) that we'd like
`is`/`!is`/`as`/`as?` to work similarly
to `===`/`!==`.

Also note that it now gives a clearer
explaination of why some corner cases work
the way they do. For example,
`FirPsiDiagnosticTestGenerated.testLambdaInLhsOfTypeOperatorCall`
yields `UNCHECKED_CAST` instead of
`CAST_NEVER_SUCCEEDS`, because
`toTypeInfo()` replaces all type arguments
with star projections, even when the argument
is not a type parameter. This is because
it has been desided to work this way in
KT-57779.

In
`FirPsiOldFrontendDiagnosticsTestGenerated..NeverSucceeds#testNoGenericsRelated`
the diagnostic is introduced, because
`t2 as FC1` and `FC1` is a final class with
no `T5` supertype.

`UNCHECKED_CAST` in
`FirPsiOldFrontendDiagnosticsTestGenerated.testSmartCast`
disappeared, because previously we didn't
take smartcasts into account.

Note that
`FirPsiOldFrontendDiagnosticsTestGenerated.testMappedSubtypes`
is a false positive. It appears because `isSubtypeOf()` doesn't
take into account platform types in supertypes of the given types
(doesn't map them).

compiler/testData/diagnostics/tests/smartCasts/alwaysNull.fir.kt

@@ -16,7 +16,7 @@ fun String?.gav() {}
 fun bar(s: String?) {
     if (s != null) return
     s.gav()
-    s as? String
+    s <!USELESS_CAST!>as? String<!>
     s as String?
-    s as String
+    s <!CAST_NEVER_SUCCEEDS!>as<!> String
 }

compiler/testData/diagnostics/tests/smartCasts/castchecks/variables.fir.kt

@@ -43,8 +43,8 @@ fun f(a: SomeClass?) {
         // 'aa' cannot be cast to SomeSubClass
         aa<!UNSAFE_CALL!>.<!>hashCode()
         aa.<!UNRESOLVED_REFERENCE!>foo<!>
-        (aa as? SomeSubClass)<!UNSAFE_CALL!>.<!>foo
-        (aa as SomeSubClass).foo
+        (aa <!USELESS_CAST!>as? SomeSubClass<!>)<!UNSAFE_CALL!>.<!>foo
+        (aa <!CAST_NEVER_SUCCEEDS!>as<!> SomeSubClass).foo
     }
     val b = (aa as? SomeSubClass)?.foo
     aa = null
@@ -52,8 +52,8 @@ fun f(a: SomeClass?) {
         // 'aa' cannot be cast to SomeSubClass
         aa<!UNSAFE_CALL!>.<!>hashCode()
         aa.<!UNRESOLVED_REFERENCE!>foo<!>
-        (aa as? SomeSubClass)<!UNSAFE_CALL!>.<!>foo
-        (aa as SomeSubClass).foo
+        (aa <!USELESS_CAST!>as? SomeSubClass<!>)<!UNSAFE_CALL!>.<!>foo
+        (aa <!CAST_NEVER_SUCCEEDS!>as<!> SomeSubClass).foo
     }
     aa = a
     val c = aa as? SomeSubClass