(rewriter.cache_all=true) refutation soundness bug

commit: 1fc9a7ba8
```
$ z3release small.smt2
sat
$ z3release rewriter.cache_all=true small.smt2
unsat
$ z3-4.8.10 rewriter.cache_all=true small.smt2
unsat
$ cat small.smt2
(declare-fun a () String)
(assert
 (str.in_re
 (str.++ a "z")
 (re.++
  (re.union (str.to_re "c") (str.to_re (ite (= a "") "z" "")))
  (str.to_re "z")
  (str.to_re (ite (= a "") "z" "")))))
(check-sat)
```