G1: Remove duplicate checks in G1BarrierSetC1::post_barrier

`G1BarrierSetC1::post_barrier` has the following checks:

```
  if (!new_val->is_register()) {
    XXX
  }
  assert(new_val->is_register(), "must be a register at this point");
  
  ...
  
  if (!new_val->is_register()) {
    YYY
  }
  assert(new_val->is_register(), "must be a register at this point");
```

The second `if` is redundant and can be removed.