[11] RFR: Assorted C2 cleanups

[Roman Kennke]

This looks good to me!

Thanks,
Roman

> 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}
>