replace barrier's input with barrier's output in all dominated uses to decrease pressure on register allocator

[Roman Kennke]

Sounds and looks good.

The changes in shenandoahHeap.cpp|hpp seem unrelated though.

Roman

Am Freitag, den 02.12.2016, 17:33 +0100 schrieb Roland Westrelin:
> http://cr.openjdk.java.net/~roland/shenandoah/registerpressure/webrev
> .00/
> 
> This implements Roman's suggestion that when we use an oop directly
> and
> there's a dominating barrier, we can safely replace the oop by the
> output of the barrier. So for instance:
> 
> a' = rb(a);
> ..
> call(a);
> 
> can also be compiled as:
> 
> a' = rb(a);
> ..
> call(a');
> 
> and if there's no use of a after the barrier then we don't keep both
> a
> and a' live but only a'.
> 
> This is implemented in the patch:
> - for write barriers at barrier expansion time.
> - for read barriers, when read barriers are scheduled.
> 
> Roland.