While rewriting some of the heap size initialization code we hit a corner-case where the setting of ShenandoahMaxRegionSize to something that isn't a power-of-2 will hit an assert in `max_heap_for_compressed_oops`.

When running with:
java -XX:+UseShenandoahGC -XX:+UnlockExperimentalVMOptions -XX:ShenandoahMaxRegionSize=33m -version

 The following code:
```
  size_t displacement_due_to_null_page = align_up(os::vm_page_size(),
                                                  _conservative_max_heap_alignment)
```
triggers:
```
# assert(is_power_of_2(alignment)) failed: must be a power of 2: 34603008
```
because `_conservative_max_heap_alignment` is not a power-of-2.

This happens because Shenandoah's `conservative_max_heap_alignment()`
 returns a potentially unaligned `ShenandoahMaxRegionSize` value.

```
size_t ShenandoahArguments::conservative_max_heap_alignment() {
  size_t align = ShenandoahMaxRegionSize;
  if (UseLargePages) {
    align = MAX2(align, os::large_page_size());
  }
  return align;
}
```

I propose a small fix to adjust `align` to be a power-of-2.