FIR DFA: fork flow everywhere

In theory, forking persistent flows should be cheap because of object
reuse, so the proposal here is to start from scratch and prove
redundancy of forks on a case-by-case basis. Something something better
safe than sorry.

^KT-28333 tag fixed-in-k2
^KT-28489 tag fixed-in-k2

compiler/fir/analysis-tests/testData/resolve/cfg/flowFromInplaceLambda2.kt

@@ -41,7 +41,7 @@ fun test4(x: String?) {
         foo(
             id(if (true) run { p = null; n() } else run { n() }),
             1,
-            run { p<!UNSAFE_CALL!>.<!>length; 123 } // Bad (p = null possible)
+            run { <!SMARTCAST_IMPOSSIBLE!>p<!>.length; 123 } // Bad (p = null possible)
         )
         p<!UNSAFE_CALL!>.<!>length // Bad (p = null possible)
     }