(propagate-bv-bounds) Refutation unsoundness on a BV instance

OS: Ubuntu 18.04
Commit: https://github.com/Z3Prover/z3/commit/7c08e53e944f806af7db74cc512a197c4d76c7f6

```
[549] % z3release small.smt2
sat
unsat
[550] % cat small.smt2
(declare-fun a () (_ BitVec 16))
(assert (not (= (_ bv1 32) ((_ zero_extend 16) a))))
(assert (bvsgt ((_ zero_extend 16) a) (_ bv1 32)))
(check-sat)
(check-sat-using (then propagate-bv-bounds smt))
```