In `ParallelCompactData::initialize_region_data`:

```
const size_t count = (region_size + RegionSizeOffsetMask) >> Log2RegionSize;
```

The `+ RegionSizeOffsetMask` part is to account for `region_size` being unaligned. However, in the sole caller of this method, the alignment is already asserted.

```
  assert((region_size & RegionSizeOffsetMask) == 0,
         "region size not a multiple of RegionSize");
```

Drop the unnecessary adjustment and move the assertion inside the method as a precondition.