Merge upstream to jdk-12+31

[Aleksey Shipilev]

On 2/11/19 2:17 PM, Roman Kennke wrote:
> I'd like to merge changes from jdk/jdk12 -> sh/jdk12 up to latest tag
> jdk-13+31. >

It must be jdk-12+31, right?

> It merged trivially without any conflicts, and testing hotspot_gc_shenandoah is all good.
> http://cr.openjdk.java.net/~rkennke/upstream-jdk12-merge-2019-02-11/changes.txt

Looks okay. At some point in the future, we would need to switch to jdk-updates/jdk12u. I have
backported quite a few Shenandoah fixes there already, and those fixes are not present in jdk/jdk12.

-Aleksey