intel/compiler: add support for fragment shading rate variable

v2: Drop old register type initializers (Jason)
    Simplify instruction snippet (Jason)

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7455>

src/intel/compiler/brw_fs.h

@@ -208,6 +208,7 @@ public:
    fs_reg *emit_samplepos_setup();
    fs_reg *emit_sampleid_setup();
    fs_reg *emit_samplemaskin_setup();
+   fs_reg *emit_shading_rate_setup();
    void emit_interpolation_setup_gfx4();
    void emit_interpolation_setup_gfx6();
    void compute_sample_position(fs_reg dst, fs_reg int_sample_pos);