RFR: couple small fixes from lucene testing

[Roland Westrelin]

http://cr.openjdk.java.net/~roland/shenandoah/lucene-fixes/webrev.00/

This should fix:

assert(n->Opcode() != Op_ShenandoahWriteBarrier || n->outcnt() > 1) failed: bad shenandoah write barrier

and

assert(dom_use->adr_type() != TypePtr::BOTTOM) failed: no bottom memory phi

crashes.

Roland.