glsl_to_nir: skip ir_var_shader_shared variables

glsl_to_nir: skip ir_var_shader_shared variables

These should be lowered away in GLSL IR but if we don't get dead
code to clean them up it causes issues in glsl_to_nir.

We wan't to drop as many GLSL IR opts in future as we can so this
makes glsl_to_nir just ignore the vars if it sees them.

In future we will want to just use the nir lowering pass that
Vulkan currently uses.

Acked-by: Elie Tournier <elie.tournier@collabora.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>