RFR [9] 2018-02-01: Bulk backports to sh/jdk9

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/backports/jdk9-20180201/webrev.01/

This backports the follow-up bugfixes we have recently found to sh/jdk9:

16198c705496: [backport] Conditionalize PerfDataMemorySize on enabled heap sampling
dd1b2cd3c66e: [backport] Make major GC phases exclusive from each other
4050463704a4: [backport] Single GCTimer shared by all operations
af9272163588: [backport] fix -ShenandoahWriteBarrierMemBar and loop unswitching

sh/jdk10 nightly is running with them now to verify separately.

Testing: hotspot_gc_shenandoah {fastdebug|release}

Thanks,
-Aleksey