intel/compiler: Add support for variable workgroup size

Add new builtin parameters that are used to keep track of the group
size.  This will be used to implement ARB_compute_variable_group_size.

The compiler will use the maximum group size supported to pick a
suitable SIMD variant.  A later improvement will be to keep all SIMD
variants (like FS) so the driver can select the best one at dispatch
time.

When variable workgroup size is used, the small workgroup optimization
is disabled as it we can't prove at compile time that the barriers
won't be needed.

Extracted from original i965 patch with additional changes by
Caio Marcelo de Oliveira Filho.

Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4504>

src/intel/compiler/brw_fs.h

@@ -370,6 +370,7 @@ public:
    int *push_constant_loc;
 
    fs_reg subgroup_id;
+   fs_reg group_size[3];
    fs_reg scratch_base;
    fs_reg frag_depth;
    fs_reg frag_stencil;