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/tests/smartCasts/safecalls/nullableReceiverWithExclExcl.fir.kt

@@ -7,5 +7,5 @@ fun test(foo: Foo?) {
         // Unnecessary
         foo<!UNNECESSARY_SAFE_CALL!>?.<!>bar?.length
     }
-    foo.bar<!UNNECESSARY_SAFE_CALL!>?.<!>length
+    foo.bar?.length
 }