tgsi_to_nir: Produce optimized NIR for a given pipe_screen.

tgsi_to_nir: Produce optimized NIR for a given pipe_screen.

With this patch, tgsi_to_nir will output NIR that is tailored to
the given pipe, by reading its capabilities and adjusting the NIR code
to those capabilities similarly to how glsl_to_nir works.

It also adds an optimization loop that brings the output NIR in line
with what glsl_to_nir outputs. This is necessary for the same reason
why glsl_to_nir has its own optimization loop: currently not every
driver does these optimizations yet.

For uses which cannot pass a pipe_screen we also keep a variant
called tgsi_to_nir_noscreen which keeps the old behavior.

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