[sh/8u] RFR 8275051: Shenandoah: Correct ordering of requested gc cause and gc request flag

[Zhengyu Gu]

I would like to backport this Shenandoah specific patch to Shenandoah 8u.

The patch corrects the order of a couple of GC flags, ensure GC sees 
latest GC cause when a GC is requested.


The original patch does not apply cleanly. The conflict is due to 
context difference in ShenandoahControlThread::handle_requested_gc() 
method, where newer version has code to handle concurrent GC breakpoint.

Original bug: https://bugs.openjdk.java.net/browse/JDK-8275051
Original patch: 
https://github.com/openjdk/jdk/commit/1ab64143c06e33e23172dd77c39e434443347364


Webrev: http://cr.openjdk.java.net/~zgu/shenandoah/JDK-8275051-8u/webrev/

Test:
   hotspot_gc_shenandoah


Thanks,

-Zhengyu