intel/fs: track more steps with INTEL_DEBUG=optimizer

intel/fs: track more steps with INTEL_DEBUG=optimizer

One particular nice thing to have is the first generated backend IR
before validation. Especially if you made a mistake in the NIR
translation, you can at least look at it before validation tells you
off.

Then the last 2 steps of the optimize() function can be interesting to
look at.

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/24552>