intel/brw: Print defs in dump_instructions

Like NIR, we print SSA defs as %1, %2, and so on.  The number here is
the VGRF number.  VGRFs that don't correspond to a SSA def remain
printed as vgrf1, vgrf2, and so on.

This makes it much easier to see what values are SSA and which aren't.

Reviewed-by: Caio Oliveira <caio.oliveira@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/28666>