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

[Zhengyu Gu]

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