intel/fs: Use OPT() for split_virtual_grfs

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header comment which is massively out-of-date.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>