FIR DFA: when breaking aliasing, keep the rest of the group together

var a = ...
  var b = a
  var c = a
  a = ...
  // b and c still aliased

compiler/testData/diagnostics/tests/smartCasts/variables/aliasing.fir.kt

@@ -79,6 +79,6 @@ fun test3() {
     if (b is String) {
         a.<!UNRESOLVED_REFERENCE!>length<!> // error
         b.length // ok
-        c.<!UNRESOLVED_REFERENCE!>length<!> // ok
+        c.length // ok
     }
 }