RFR: [8u] Bulk backports to sh/jdk8u

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/backports/jdk8u-20171207/webrev.01/

This backports current work to make AArch64 perform well.

[backport] Check BS type in immByteMapBase predicate
[backport] Optimize oop/fwdptr/hr_index verification a bit
[backport] Optimize fwdptr region handling in ShenandoahVerifyOopClosure::verify_oop
[backport] SieveObjects test is too hostile to verification

Testing: hotspot_gc_shenandoah {fastdebug|release}

Thanks,
-Aleksey