glsl/cs: Don't strip gl_GlobalInvocationID and dependencies

glsl/cs: Don't strip gl_GlobalInvocationID and dependencies

We lower gl_GlobalInvocationID based on the extension spec formula:

    gl_GlobalInvocationID =
        gl_WorkGroupID * gl_WorkGroupSize + gl_LocalInvocationID

https://www.opengl.org/registry/specs/ARB/compute_shader.txt

We need to set this variable in main(), even if gl_GlobalInvocationID
is not referenced by the shader. (It may be used by a linked shader.)
Therefore, we can't eliminate these as dead variables.

Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>