Remove bogus DCHECK in BCE.

When creating a phi for the array length when we add HDeoptimization
nodes, we might update accesses in inner loops to use that phi instead
of the array length. The BCE phase was not expecting this case.

Bug: 21034044

(cherry picked from commit 3cde6227)

Change-Id: I639f4ea6f5889726142041a42736183f162c7437