RFR 8225341: Shenandoah: CM::update_thread_roots() needs to handle derived pointers

[Aleksey Shipilev]

On 6/5/19 2:38 PM, Zhengyu Gu wrote:
> Update_thread_roots() may alter stack frames.
> 
> Bug: https://bugs.openjdk.java.net/browse/JDK-8225341
> Webrev: http://cr.openjdk.java.net/~zgu/JDK-8225341/webrev.00/

I think we should strive to use COMPILER2_OR_JVMCI macro, see
ShenandoahMarkCompact::phase3_update_references. The remaining uses of older define should be
cleaned up some time later.

Otherwise looks good.

-Aleksey