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

[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.