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>

docs/gallium/screen.rst

@@ -412,9 +412,6 @@ The integer capabilities:
   ARB_transform_feedback3.
 * ``PIPE_CAP_SHADER_CAN_READ_OUTPUTS``: Whether every TGSI shader stage can read
   from the output file.
-* ``PIPE_CAP_GLSL_OPTIMIZE_CONSERVATIVELY``: Tell the GLSL compiler to use
-  the minimum amount of optimizations just to be able to do all the linking
-  and lowering.
 * ``PIPE_CAP_FBFETCH``: The number of render targets whose value in the
   current framebuffer can be read in the shader.  0 means framebuffer fetch
   is not supported.  1 means that only the first render target can be read,