Shenandoah: SBSA::load_at() should save/restore registers when calling SATB barrier

[Zhengyu Gu]

In SBSA::load_at(), it calls shenandoah_write_barrier_pre() for 
references that need to be kept alive. However, the call has 
side-effects that may mess up registers. Especially, the slow-path 
contains runtime call.

This problem impacts c2i_entry_barrier(), where upstream uses 
resolve_weak_handle() which keeps the handle alive, while Shenandoah 
uses peek_weak_handle(), which does not keep the handle alive, so it 
bypasses SATB barrier. I think this just hides the problem.

Shenandoah should just save and restore register around 
shenandoah_write_barrier_pre() call to eliminate register issues, hence, 
it can fallback to upstream c2i_entry_barrier() implementation and 
eliminate the need for peek_weak_handle().

This fix also fixes the intermittent is_loader_alive() assertion we have 
seen in nightly tests. While this assertion is relative rare on x86_64, 
but it is quite reproducible on x86_32 (because of it has less 
registers(?)). With this fix, I have yet seen once.


Webrev: 
http://cr.openjdk.java.net/~zgu/shenandoah/load_at_keepalive_fix/webrev.00/

Test:
   hotspot_gc_shenandoah (fastdebug and release) x86_64 and x86_32 on Linux

Thanks,

-Zhengyu