RFR: Remove C2 write-barrier from .ad files

[Aleksey Shipilev]

On 07/09/2018 06:07 PM, Roman Kennke wrote:
> I propose to get rid of write-barriers in C2 .ad files, and instead
> always expand WBs in ideal graph. We've been doing this for quite a
> while now by default, with seemingly no ill effects.
> 
> In next step, this will make it easier to move
> MacroAssembler::shenandoah_write_barrier() away into gc/shenandoah
> subfolder.
> 
> http://cr.openjdk.java.net/~rkennke/remove-c2-wb/webrev.01/

That makes sense, looks good. Would it make sense to leave the ShenandoahWriteBarrier matcher with
ShouldNotReachHere()? Or are we crashing the compilation reliably (e.g. in release builds?) when
Ideal is not matched?

Thanks,
-Aleksey