RFR: Optimize oop/fwdptr/hr_index verification a bit

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/verifier-perf-hr/webrev.01/

This slightly optimizes verification code, which should make fastdebug builds run a bit faster.

Testing: hotspot_gc_shenandoah

Thanks,
-Aleksey