message: memory: merge undef vars when merging memories
diff: @@ -1720,6 +1720,7 @@ Memory Memory::mkIf(const expr &cond, const Memory &then, const Memory &els) {
  if (els.escaped_local_blks[i])
  ret.escaped_local_blks[i] = true;
  }
  + ret.undef_vars.insert(els.undef_vars.begin(), els.undef_vars.end());
  return ret;
  }
  