RFR (S) 8241748: Shenandoah: inline MarkingContext TAMS methods

[Aleksey Shipilev]

RFE:
  https://bugs.openjdk.java.net/browse/JDK-8241748

ShenandoahMarkingContext methods that deal with TAMS are accessed on hot paths. These should be inlined.

Webrev:
  https://cr.openjdk.java.net/~shade/8241748/webrev.01/

Testing: hotspot_gc_shenandoah; Linux x86_64 {slowdebug, fastdebug, release} builds without PCH

-- 
Thanks,
-Aleksey