[FIR] Ignore nullability in the definition of "identity-less"

The definition of a "builtin" already
ignores nullability.

`Int? === String?` must trigger a
diagnostic by design.

compiler/tests-spec/testData/diagnostics/notLinked/dfa/neg/1.fir.kt

@@ -122,7 +122,7 @@ fun case_9(x: TypealiasNullableString<!REDUNDANT_NULLABLE!>?<!>) {
 fun case_10() {
     val a = Class()
 
-    if (a.prop_4 === null || <!USELESS_IS_CHECK!>true is Boolean<!>) {
+    if (<!FORBIDDEN_IDENTITY_EQUALS_WARNING!>a.prop_4 === null<!> || <!USELESS_IS_CHECK!>true is Boolean<!>) {
         if (<!FORBIDDEN_IDENTITY_EQUALS_WARNING, SENSELESS_COMPARISON!>a.prop_4 != null !== null<!>) {
             a.prop_4
             a.prop_4<!UNSAFE_CALL!>.<!>equals(null)