nir: Accept nir_var_mem_global in derefs used by phis
This mode is used by PhysicalStorageBufferEXT storage class.
Fixes: 8bdf5a00 "nir: Allow derefs to be used as phi sources"
This mode is used by PhysicalStorageBufferEXT storage class.
Fixes: 8bdf5a00 "nir: Allow derefs to be used as phi sources"