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>

src/glsl/opt_dead_builtin_variables.cpp

@@ -62,6 +62,16 @@ optimize_dead_builtin_variables(exec_list *instructions,
        * information, so removing these variables from the user shader will
        * cause problems later.
        *
+       * For compute shaders, gl_GlobalInvocationID has some dependencies, so
+       * we avoid removing these dependencies.
+       *
+       * We also avoid removing gl_GlobalInvocationID at this stage because it
+       * might be used by a linked shader. In this case it still needs to be
+       * initialized by the main function.
+       *
+       *    gl_GlobalInvocationID =
+       *       gl_WorkGroupID * gl_WorkGroupSize + gl_LocalInvocationID
+       *
        * Matrix uniforms with "Transpose" are not eliminated because there's
        * an optimization pass that can turn references to the regular matrix
        * into references to the transpose matrix.  Eliminating the transpose
@@ -73,6 +83,10 @@ optimize_dead_builtin_variables(exec_list *instructions,
        */
       if (strcmp(var->name, "gl_ModelViewProjectionMatrix") == 0
           || strcmp(var->name, "gl_Vertex") == 0
+          || strcmp(var->name, "gl_WorkGroupID") == 0
+          || strcmp(var->name, "gl_WorkGroupSize") == 0
+          || strcmp(var->name, "gl_LocalInvocationID") == 0
+          || strcmp(var->name, "gl_GlobalInvocationID") == 0
           || strstr(var->name, "Transpose") != NULL)
          continue;