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.fir.txt

@@ -74,7 +74,7 @@ FILE: flowFromInplaceLambda2.kt
                     }
                 }
                 ), Int(1), R|kotlin/run|<R|kotlin/Int|>(<L> = run@fun <anonymous>(): R|kotlin/Int| <inline=Inline, kind=EXACTLY_ONCE>  {
-                    R|<local>/p|.<Inapplicable(UNSAFE_CALL): kotlin/String.length>#
+                    R|<local>/p|.<Inapplicable(UNSTABLE_SMARTCAST): kotlin/String.length>#
                     ^ Int(123)
                 }
                 ))