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/varnotnull/toFlexibleType.fir.kt

@@ -9,8 +9,8 @@ class J {
 fun bar() {
     var v: String?
     v = J.foo()
-    v.length
-    gav(v)
+    v<!UNSAFE_CALL!>.<!>length
+    <!INAPPLICABLE_CANDIDATE!>gav<!>(v)
 }
 
-fun gav(v: String) = v
+fun gav(v: String) = v