RFR: 8218767: ZGC: Do not assume that r12 is a special register in C2

[Per Liden]

Updated version rebased on top of JDK-8217909 (which Roman will push 
soonish).

http://cr.openjdk.java.net/~pliden/8218767/webrev.1

cheers,
Per

On 2/19/19 2:40 PM, Per Liden wrote:
> The C2 load barrier code for ZGC assumes that r12 is not used as a 
> general purpose register. However, JDK-8217909 might change that, so ZGC 
> should not be making this assumption.
> 
> Bug: https://bugs.openjdk.java.net/browse/JDK-8218767
> Webrev: http://cr.openjdk.java.net/~pliden/8218767/webrev.0
> 
> /Per