Do not look at dead phis during SsaRedundantPhiElimination.

Otherwise, we may replace a dead loop phi with its incoming input.
This broke an assumption during liveness analysis in the presence
of irreducible loops.

bug:28256552

(cherry picked from commit 05b3fa02)

Change-Id: I297c8fbd9a2414dd852aa932595f7b42d8f1a584