RFR: rework split if

[Roman Kennke]

Oh yeah, this looks much better than what I did back then.

Please push! And I suppose you'll upstream the shared parts soon?

Thanks,
Roman

> http://cr.openjdk.java.net/~roland/shenandoah/split_if/webrev.00/
> 
> This reworks how split if handles the write barrier:
> 
> - it makes split if trigger more reliably so could result in some
>   performance improvements
> 
> - it fixes handling of write barrier projection
> 
> Roland.
>