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/testData/diagnostics/testsWithStdLib/smartcasts/withChangesToNull.fir.kt

@@ -3,7 +3,7 @@ fun foo(y: String?) {
     if (x != null) {
         with(y?.let { x = null; it }) {
             this<!UNSAFE_CALL!>.<!>length
-            x<!UNSAFE_CALL!>.<!>length
+            <!SMARTCAST_IMPOSSIBLE!>x<!>.length
         }
         x<!UNSAFE_CALL!>.<!>length
     }