intel/fs: Use OPT() for split_virtual_grfs

Now that we're being conservative in the pass, it's easy to tell when it
makes progress and we can put it in the OPT() macro.  This way, we get
nice INTEL_DEBUG=optimizer dumps for it.  While we're here, fix the
header comment which is massively out-of-date.

Reviewed-by: Emma Anholt <emma@anholt.net>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/13734>

src/intel/compiler/brw_fs.h

@@ -148,7 +148,7 @@ public:
    void assign_regs_trivial();
    void calculate_payload_ranges(int payload_node_count,
                                  int *payload_last_use_ip) const;
-   void split_virtual_grfs();
+   bool split_virtual_grfs();
    bool compact_virtual_grfs();
    void assign_constant_locations();
    bool get_pull_locs(const fs_reg &src, unsigned *out_surf_index,