Relax the DCHECK in load store elimination.

The DCHECK was too strong, as we could come from a field or array
get on null, instead of null directly.

bug:27831001

(cherry picked from commit 65fef30952bb92acec7ed36f7f431d93f7ce88b3)

Change-Id: Ia3ba1235e95408d66349a02fc438df9c2cf9e255
2 files changed