[FIR] Prohibit is-checks and casts for unrelated nullable types

This decision was made after a discussion
in Slack.

compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/checkers/expression/FirCastOperatorsChecker.kt

@@ -75,7 +75,10 @@ object FirCastOperatorsChecker : FirTypeOperatorCallChecker(MppCheckerKind.Commo
 
         return when {
             isRefinementUseless(context, l.directType.upperBoundIfFlexible(), r.directType, expression) -> useless
-            oneIsNotNull && shouldReportAsPerRules1(l, r, context) -> impossible
+            shouldReportAsPerRules1(l, r, context) -> when {
+                oneIsNotNull -> impossible
+                else -> useless
+            }
             isCastErased(l.directType, r.directType, context) -> Applicability.CAST_ERASED
             else -> Applicability.APPLICABLE
         }

compiler/testData/diagnostics/tests/cast/NullableToNullable.fir.kt

@@ -0,0 +1,7 @@
+// From KT-13324: always succeeds
+val x = null as String?
+// From KT-260: sometimes succeeds
+fun foo(a: String?): Int? {
+    val c = a <!USELESS_CAST!>as? Int?<!>
+    return c
+}

compiler/testData/diagnostics/tests/cast/NullableToNullable.kt

@@ -1,4 +1,3 @@
-// FIR_IDENTICAL
 // From KT-13324: always succeeds
 val x = null as String?
 // From KT-260: sometimes succeeds