RFR: Make C1 write-barrier use a stub instead of dedicated instruction

[Aleksey Shipilev]

On 07/03/2018 12:50 PM, Roman Kennke wrote:
> We currently use a dedicated C1 LIR instruction to implement the
> Shenandoah write barrier. This is not easily supported by the current
> BarrierSetC1 interface. What works better is to use CodeStub (like all
> other GC's barriers), so let's turn Shenandoah's write barrier (back) to
> using CodeStubs. This removes a whole lot of upstream changes and
> confines Shenandoah barrier stuff to gc/shenandoah.
> 
> http://cr.openjdk.java.net/~rkennke/c1-shenandoah-codestubs/webrev.01/

*) Comment is wrong in ShenandoahBarrierSetC1::write_barrier_impl:

 224   // Read the marking-in-progress flag.

Otherwise looks good!

-Aleksey