RFR: 8236181: C2: Remove useless step_over_gc_barrier() in int->bool conversion

[Roland Westrelin]

> However, it is not needed because Shenandoah we already eliminate
> barriers when the only user is a null-check, and it might actually be
> counter-productive if the barrier is needed on other paths, because it
> keeps the input of the barrier alive. This is probably a left-over
> from pre-LRB.

I'm not sure this is useless. If this triggers during igvn, it could be
that the Phi is processed before the CmpP and so the barrier at the CmpP
has no chance of being eliminated. Anyway, this is also likely uncommon
so I'm fine with removing it.

Roland.