Remove GLSL dependency from Gallium by carving out nir_lower_samplers from libglsl

Previously it was only possible to produce NIR that drivers could actually use if TTN (TGSI to NIR) called gl_nir_lower_samplers or gl_nir_lower_samplers_as_deref. This added an undesired dependency on libglsl. This MR fixes that by carving out the non-GL-specific functionality from libglsl into a function named nir_lower_samplers and removing the call to gl_nir_lower_samplers_as_deref. TTN now manually counts the textures used instead of relying on that GLSL function.