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

[Roman Kennke]

Am Freitag, den 02.12.2016, 18:35 +0100 schrieb Roland Westrelin:
> > The changes in shenandoahHeap.cpp|hpp seem unrelated though.
> 
> They are. At some point I tried building an optimized build and those
> changes were required.

Hmm, strange. Doesn't matter, please push!

Roman