RFR: Improve/trim register saving/restoring around WB slowpath call in stub

[Aleksey Shipilev]

On 11/20/18 9:23 PM, Roman Kennke wrote:
> I don't think it's doing much performance wise because it's the
> slow-path anyway, and vector saving/restoring (32*8 words) would drown
> it anway, but it's slightly nicer too I think. I'll not push for it, if
> not considered useful.

It might be useful if it simplifies and streamlines things. Death by a thousand cuts is real.

> Testing: tier3_gc_shenandoah, specjvm+aggressive
> 
> http://cr.openjdk.java.net/~rkennke/improve-wb-stub/webrev.00/

Can you rebase it to current Shenandoah, please?

Thanks,
-Aleksey