Fix SsaDeadPhiElimination in the presence of dependent phis.

This fixes the problem of having a dead loop phi taking as back-edge
input a phi that also has this loop phi as input. Walking backwards
does not solve the problem because the loop phi will be visited last.

Most of the time, dex removes dead locals like this.

Change-Id: I797198cf9c15f8faa6585cca157810e23aaa4940
3 files changed