FIR: Report UNSAFE_CALL instead of SMARTCAST_IMPOSSIBLE when smartcast

to null.

Currently the error message shows the expression is impossible to
smartcast to the nullable type, which is nonsensical since it's already
that type.

compiler/fir/analysis-tests/testData/resolve/smartcasts/nullability.kt

@@ -175,3 +175,10 @@ fun test_13(q: QImplMutable?) {
         <!SMARTCAST_IMPOSSIBLE!>q.data<!>.s.inc() // should be bad
     }
 }
+
+fun test_14(q: Q) {
+    // `q.data` is a property that has an open getter
+    if (q.data == null) {
+        q.data<!UNSAFE_CALL!>.<!>s // should be UNSAFE_CALL and NOT SMARTCAST_IMPOSSIBLE
+    }
+}