glsl: Make all drivers take the GLSLOptimizeConservatively path.

glsl: Make all drivers take the GLSLOptimizeConservatively path.

Now that all consumers of GLSL use NIR, make the remaining drivers take
the path that relies on NIR to really do optimization.

nouveau steam shader-db runtime -6.69631% +/- 1.29235% (n=12).
No change on shader-db there.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Karol Herbst <kherbst@redhat.com>
Reviewed-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16364>