intel/cs: Push subgroup ID instead of base thread ID

We're going to want subgroup ID for SPIR-V subgroups eventually anyway.
We really only want to push one and calculate the other from it.  It
makes a bit more sense to push the subgroup ID because it's simpler to
calculate and because it's a real API thing.  The only advantage to
pushing the base thread ID is to avoid a single SHL in the shader.

Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>

src/intel/compiler/brw_fs.h

@@ -315,7 +315,7 @@ public:
     */
    int *push_constant_loc;
 
-   fs_reg thread_local_id;
+   fs_reg subgroup_id;
    fs_reg frag_depth;
    fs_reg frag_stencil;
    fs_reg sample_mask;