message: smt: add fold (sext a) == 0 -> a == 0
diff: @@ -1339,9 +1339,15 @@ expr expr::cmp_eq(const expr &rhs, bool simplify) const {
  if (isURem(a, b) && rhs.uge(b).isTrue())
  return false;
  + if (isSignExt(a)) {
  // (sext a) == (sext b) -> a == b
  - if (isSignExt(a) && rhs.isSignExt(b))
  + if (rhs.isSignExt(b))
  return a == b;
  +
  + // (sext a) == 0 -> a == 0
  + if (b.isZero())
  + return a == mkUInt(0, a);
  + }
  }
  end:
  