[FIR] Move check for _private-to-this_ visibility into checker

^KT-55446
^KT-65790 Fixed

compiler/testData/diagnostics/tests/visibility/privateToThisExotic.fir.kt

@@ -20,7 +20,6 @@ class Foo<in T> : Base<<!TYPE_VARIANCE_CONFLICT_ERROR!>T<!>>() {
 
     fun bar(f: Foo<Bar>) {
         val dnn = f.<!INVISIBLE_REFERENCE!>dnn<!>
-        // This case (and any other with non-denotable type) requires KT-55446 to be fixed
-        val flex = f.flex
+        val flex = f.<!INVISIBLE_REFERENCE!>flex<!>
     }
 }