RFR: Hide is_concurrent_traversal_in_progress check under the type check

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/alloc-hide-ha/webrev.01/

Micro-optimization: this avoid ShHeap field access on the usual path, instead checking on local
variable.

Testing: hotspot_gc_shenandoah

Thanks,
-Aleksey