Do not place null check from unresolved field access.

Rationale:
These accesses go though the runtime anyway where various
checks are done, including null check. Since particular
checks, like access checks, need to occur prior to the
null check (to ensure link errors are not masked by
a null reference), the explicit null check should not
occur in the HIR.

BUG=29068831

Change-Id: I30fc9cb8cf4993e4176e235ceba3a38aef98d503