RFR: Remove obsolete and unused reduce-storeval-barrier optimization code

[Roman Kennke]

Don't bother reviewing this. I made a mistake. I removed the newval in 
some cases, but we do need it for the matrix barrier that is part of our 
C2 pre-barrier code path. I'll post an updated webrev soon (not today I 
guess... it's late here and almost weekend...)

Roman

> This is an almost 1:1 revert of an earlier changeset that attempted to 
> optimize storeval barriers. The idea back then was that when we check 
> for conc-mark-in-progress anyway, we could just as well only invoke 
> the storeval (i.e. read) - barrier if conc-mark is active. It never 
> worked very well (and we never turned it on by default) and gained us 
> anything significant. But now it's even worse: updating-refs now also 
> happens outside conc-mark, and thus the whole idea is obsolete. In 
> fact, maybe we can do something smarter once all the X_in_progress 
> flags have been folded into the gc-phase-field (need to see this when 
> it's there). The patch removes the code, and thus reduces our upstream 
> diff.
>
> Test: hotspot_gc_shenandoah
>
> http://cr.openjdk.java.net/~rkennke/remove-reduce-storeval/webrev.00/ 
> <http://cr.openjdk.java.net/%7Erkennke/remove-reduce-storeval/webrev.00/>
>
> Ok?
>