[FIR] Add REDUNDANT_NULLABLE diagnostics

compiler/tests-spec/testData/diagnostics/notLinked/dfa/pos/19.fir.kt

@@ -1,6 +1,7 @@
 // !LANGUAGE: +NewInference
 // !DIAGNOSTICS: -UNUSED_EXPRESSION
 // SKIP_TXT
+// WITH_EXTENDED_CHECKERS
 
 // TESTCASE NUMBER: 1
 fun case_1(x: Any?) {
@@ -68,7 +69,7 @@ fun case_8(x: Any?) {
 
 // TESTCASE NUMBER: 9
 fun case_9(x: Any?) {
-    if (!!(x !is TypealiasNullableStringIndirect?)) else {
+    if (!!(x !is TypealiasNullableStringIndirect<!REDUNDANT_NULLABLE!>?<!>)) else {
         <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Any? & TypealiasNullableStringIndirect?")!>x<!>
         <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Any? & TypealiasNullableStringIndirect?")!>x<!>?.get(0)
     }