FIR DFA: revalidate reassigned variables after loops

If a certain type statement is true on loop entry and all continue
paths, then it is also true on exit if the condition did not reassign
the variable.

^KT-7676 tag fixed-in-k2

compiler/testData/diagnostics/tests/smartCasts/variables/whileWithBreak.fir.kt

@@ -13,5 +13,5 @@ fun list(start: String) {
         e = e.next()
     }
     // e can never be null but we do not know it
-    e<!UNSAFE_CALL!>.<!>hashCode()
+    e.hashCode()
 }