RFR 8233165: Shenandoah:SBSA::gen_load_reference_barrier_stub() should use pointer register for address on aarch64

[Roman Kennke]

Nice.

Please push the fix.

Thanks,
Roman

> The load address can come in in single-size or double-size register,
> as_pointer_register() can deal with both case.
> 
> Bug: https://bugs.openjdk.java.net/browse/JDK-8233165
> Webrev: http://cr.openjdk.java.net/~zgu/JDK-8233165/webrev.00/
> 
> Test:
>   hotspot_gc_shenandoah (fastdebug and release)
>   jcstress quick tests (fastdebug and release)
>   on aarch64 Linux
> 
> Thanks,
> 
> -Zhengyu
>