RFR: 8217747: ZGC: Minor optimization of ZLoadBarrierStubC1

[Per Liden]

In the C1 load barrier stub for ZGC we currently save/restore rax unless 
it's the stub result register. However, we can also avoid this 
save/restore of rax if it's the tmp register. Avoiding a push/pop in the 
stub isn't really an important optimization, but since we already do it 
for the result register we can just as well do it for the tmp register 
too, without making the code more complex.

Bug: https://bugs.openjdk.java.net/browse/JDK-8217747
Webrev: http://cr.openjdk.java.net/~pliden/8217747/webrev.0

/Per