FIR DFA: deprioritize branches with no reassignments in flow unions

callBothLambdas({ x = "..." }, { x is Int })
  // the assignment always executes, so x is String | (String & Int);
  // the latter is always a subtype of the former so it can be ignored

compiler/fir/analysis-tests/testData/resolve/cfg/flowFromTwoInplaceLambdas.kt

@@ -38,8 +38,8 @@ fun test2(x: Any?) {
     var p: Any? = x
     p.<!UNRESOLVED_REFERENCE!>length<!> // Bad
     run2({ p = null; n() }, { p as String; 123 })
-    p<!UNSAFE_CALL!>.<!>length // Bad: p can be null
-    p?.length // OK: p is String | Nothing? = String?
+    p.<!UNRESOLVED_REFERENCE!>length<!> // Bad: p is Nothing? | (String & Nothing?) = Nothing?
+    p?.<!UNRESOLVED_REFERENCE!>length<!> // Technically OK because p is null, but what is "length"?
 }
 
 fun test3(x: Any?) {
@@ -63,3 +63,11 @@ fun test4(x: Any?) {
     x.x // OK: x is I1 & I2
     x.y // OK: x is I1 & I2
 }
+
+fun test5(x: Any?, q: String?) {
+    var p: Any? = x
+    p.<!UNRESOLVED_REFERENCE!>length<!> // Bad
+    run2({ p as Int; 123 }, { p = q; n() })
+    p<!UNSAFE_CALL!>.<!>length // Bad: p is String? | (String? & Int) = String?
+    p?.length // OK: p is String?
+}