RFR: Remove C2 leftovers

[Roman Kennke]

Duh.

1. I forgot the actual webrev.
2. We can't easy remove cast_to_nonconst().

 Here's the actual reduced patch:

http://cr.openjdk.java.net/~rkennke/remove-leftovers/webrev.01/

Ok?

Roman


> - a bunch of lone includes
> - a !UseShenandoahGC that was added when we wanted barriers on
> constants, which we don't do anymore.
> - cast_to_nonconst() which I believe was also added for supporting
> barriers on constants (and there was meant to prevent replacing a
> barrier with it's underlying constant). I don't think we need this
> anymore, and it might actually be harmful. We ensure that constants are
> always to-space.
> 
> It passes tier3_gc_shenandoah
> 
> Ok?
> 
> Roman
>