Shenandoah: fix an incorrect assert

This is on behalf of wattsun@tencent.com

assert(alias = Compile::AliasIdxRaw, "following raw memory can't lead to a barrier");
```
bool ShenandoahBarrierC2Support::is_dominator(Node* d_c, Node* n_c, Node* d, Node* n, PhaseIdealLoop* phase) {
  if (d_c != n_c) {
    return phase->is_dominator(d_c, n_c);
  }
  return is_dominator_same_ctrl(d_c, d, n, phase);
}

Node* next_mem(Node* mem, int alias) {
  Node* res = NULL;
  if (mem->is_Proj()) {
    res = mem->in(0);
  } else if (mem->is_SafePoint() || mem->is_MemBar()) {
    res = mem->in(TypeFunc::Memory);
  } else if (mem->is_Phi()) {
    res = mem->in(1);
  } else if (mem->is_MergeMem()) {
    res = mem->as_MergeMem()->memory_at(alias);
  } else if (mem->is_Store() || mem->is_LoadStore() || mem->is_ClearArray()) {
    assert(alias = Compile::AliasIdxRaw, "following raw memory can't lead to a barrier");
    res = mem->in(MemNode::Memory);
  } else {
#ifdef ASSERT
    mem->dump();
#endif
    ShouldNotReachHere();
  }
  return res;
}
```