ART: Revisit users in phi elimination

SSA phi elimination visits phis in post order so that loop phis are
visited after their inputs. This prevents elimination of phis with
other phi inputs, exacerbated by the fact that the SSA builder does
create catch phis even if all inputs are the same (unlike with normal
phis). This patch revisits phi users of eliminated phis until no more
phis can be removed.

Change-Id: I403614dd46a8e6f0a5b9dd9e8ddc8832617521eb