nir/validate: Allow derefs in phi nodes

nir/validate: Allow derefs in phi nodes

We added this assert when first moving derefs over to instructions to
ensure that deref chains could go all the way back to the variables.
Now that we're going to start using derefs for things that we can do
variable pointers on such as UBOs and SSBOs, we need to be able to run
derefs through phi nodes, selects, and basically anything else.

Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>
Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>