Fix another case of live_in at irreducible loop entry.

GVN was implicitly extending the liveness of an instruction across
an irreducible loop.

Fix this problem by clearing the value set at loop entries that contain
an irreducible loop.

bug:28252896

(cherry picked from commit 77ce6430af2709432b22344ed656edd8ec80581b)

Change-Id: Ie0121e83b2dfe47bcd184b90a69c0194d13fce54
12 files changed