Shenandoah: assert ShenandoahHeap::cas_oop addresses are aligned

Current ShenandoahHeap::cas_oop routines perform CASes on given address, hoping the hardware would handle it properly. In most cases, this is guaranteed by callers who pass aligned addresses to it. However, we should assert this directly, to catch bugs. This would fail the asserts with proper message rather than obscure SIGBUS on some platforms like AArch64.