FIR: fix nullability computation of intersection of flexible types

Without this, currently,

  it(ft(J..J?), ft(J..J?) => J

which should be ft(J..J?) instead.

compiler/testData/diagnostics/tests/smartCasts/smartcastToNothingAfterCheckingForNull.fir.kt

@@ -23,7 +23,7 @@ fun g(x: B<Int>) {
     }
 
     if (y is Nothing?) {
-        <!OVERLOAD_RESOLUTION_AMBIGUITY!>f<!>(y)
-        <!OVERLOAD_RESOLUTION_AMBIGUITY!>g<!>(y)
+        f(y)
+        <!NONE_APPLICABLE!>g<!>(y)
     }
-}
+}