FIR DFA: fix logic clear aliasing

The logic should clear back aliases as well. To ensure all back aliases
don't lose any information, statements on the original variable are
copied over to its aliases.

compiler/testData/diagnostics/tests/smartCasts/varnotnull/boundInitializerWrong.fir.kt

@@ -5,7 +5,7 @@ fun foo() {
     val y = x
     x = null
     if (y != null) {
-        x.hashCode()
+        x<!UNSAFE_CALL!>.<!>hashCode()
     }
 }