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/loops/safeCallBreakInsideDoWhile.fir.kt

@@ -8,7 +8,7 @@ fun calc(x: String?, y: String?): Int {
         // x is not null in condition but we do not see it yet
     } while (x.length > 0)
     // y is nullable because of break
-    y.length
+    y<!UNSAFE_CALL!>.<!>length
     // x is not null, at least in theory
     return x.length
 }

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
 }

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
     }