Revert "nir: assert that nir_lower_tex runs after lowering derefs"

Revert "nir: assert that nir_lower_tex runs after lowering derefs"

This reverts commit 4cda61f11e922fb5914ae73d22cc0c495abf0377 for now,
as it appears to break i965 CI (32,000+ failures).  Rob and I suspect
we need to do the equivalent of 1c6a2efa06e9bb5914f4557118930fc61065a467
on i965 - we are doing nir_lower_tex and brw_nir_lower_resources in the
wrong order and that's likely triggering this condition.  Once we fix
that, we should put this patch back.