message: smt expr: extend extract fold
diff: @@ -1573,7 +1573,7 @@ expr expr::extract(unsigned high, unsigned low) const {
  return a.extract(high - b_bw, low - b_bw);
  if (low == 0)
  return a.extract(high - b_bw, 0).concat(b);
  - if (a.isConst())
  + if (a.isConst() || b.isConst())
  return a.extract(high - b_bw, 0).concat(b.extract(b_bw-1, low));
  }
  }
  