hash collisions in program cache don't seem relevant

[nunoplopes]

Tried a better hash:

#define HASH_COMBINE(hash, ty, v) hash * 31 + std::hash<ty>()(v)
  for (unsigned i = 0; i < key.num_ops; ++i) {
    hash = HASH_COMBINE(hash, uint16_t, key.ops[i].id);
    for (auto &arg : key.ops[i].args) {
      if (auto *v = get_if<int64_t>(&arg)) {
        hash = HASH_COMBINE(hash, int64_t, *v);
      } else if (auto *v = get_if<vector<long>>(&arg)) {
        for (auto n : *v) {
          hash = HASH_COMBINE(hash, long, n);
        }
      }
    }
  }
#undef HASH_COMBINE

Reduced collisions to almost zero, but zero perf difference.