[11] RFR: Assorted C2 cleanups

[Aleksey Shipilev]

Assorted cleanups in sh/jdk11:
  http://cr.openjdk.java.net/~shade/shenandoah/11u-cleanup-c2-2/webrev.01/

*) Eliminate expanded calls to BarrierSetC2::enqueue_useful_gc_barrier
  BarrierSetC2::enqueue_useful_gc_barrier does this for us, no need to have expanded copies of it. I
opted to protect one call with UseShenandoahGC, because I am not sure it is safe for ZGC.

*) Eliminate LibraryCallKit::make_unsafe_address additions, not needed post-LRB
 Obviously not needed after LRB. is_store argument is not even used on any path.

*) Eliminate complete TODO in Compile::remove_useless_nodes
 BarrierSetC2::eliminate_useless_gc_barriers does this for us.

*) Revert useless field init reordering, field declaration order is in effect
 C++ calls constructors of class members in their declaration order, regardless of the order in the
initialization list. So this change is not needed, and it matches upstream better.

*) Remove superfluous includes in C2
 Remove Shenandoah includes where not needed. There are two places where Shenandoah headers got the
shared C2 barrier set transitively, needed to add those instead.

Testing: hotspot_gc_shenandoah {fastdebug,shenandoah}

-- 
Thanks,
-Aleksey