RFR: Excessive asserts in marked_object_iterate

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/asserts-marked-excess/webrev.01/

We don't need five asserts, when we can have only two:
  - is_null is getting implicitly checked with is_oop
  - is_in is checked implicitly with is_oop
  - (bitmap == complete) is implied by current calling code

Makes fastdebug builds a bit faster.

Testing: hotspot_gc_shenandoah

Thanks,
-Aleksey