tgsi_to_nir: use sampler variables and derefs

tgsi_to_nir: use sampler variables and derefs

v2: fix is_shadow, is_array and txq

Some drivers (eg. iris) need the presence of sampler variables and derefs
so that they can count them to determine the number of samplers used.
This change also makes the output NIR closer to what glsl_to_nir outputs.

Signed-Off-By: Timur Kristóf <timur.kristof@gmail.com>
Tested-by: Andre Heider <a.heider@gmail.com>
Tested-by: Rob Clark <robdclark@gmail.com>
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Reviewed-by: Eric Anholt <eric@anholt.net>