FIR DFA: when joining flows, clear reassigned vars with no statements

For example:

    val c = C("...")
    val x = c.x // alias to variable for c.x, which depends on c
    if (x == null) return
    // now c.x has type Any
    c = C(null)
    // c has been reassigned => info for c.x is no longer valid;
    // c itself has never had any statements made about it, but
    // we must still call removeAllAboutVariable to clear the
    // dependents

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

@@ -36,5 +36,5 @@ fun test3(p: Boolean) {
         c = C(null)
     }
     x.length // ok
-    c.x.length // bad
+    c.x<!UNSAFE_CALL!>.<!>length // bad
 }