i965: Use NIR-based clip/cull lowering for OpenGL as well.

i965: Use NIR-based clip/cull lowering for OpenGL as well.

The old approach works fine, and this approach isn't necessarily better.
But it at least has the advantage that Vulkan and GL use the same
approach.  I originally wrote it to gain additional testing for the
new paths.

shader-db statistics show 0 instruction count changes.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>