glsl: use NIR function inlining for drivers that use glsl_to_nir()

glsl: use NIR function inlining for drivers that use glsl_to_nir()

glsl_to_nir() is still missing support for converting certain
functions to NIR, so for those we use the GLSL IR optimisations
to remove the functions.

Reviewed-by: Eric Anholt <eric@anholt.net>