linker: Calculate used samplers and shadow samplers in the linker

linker: Calculate used samplers and shadow samplers in the linker

It used to be done in ir_to_mesa, and that was kind of a bad place.

I didn't change st_glsl_to_tgsi because there is some strange stuff
happening in the code that generates glDrawPixels shaders.  It looked
like this would break horribly if I touched anything.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>