i965: Sweep NIR after linking phase to free held memory

After optimization passes and many trasfromations most of memory
NIR holds is a garbage which was being freed only after shader deletion.
Freeing it at the end of linking will save memory which would be useful
in case there are a lot of complex shaders being compiled.
The common case for this issue is 32bit game running under Wine.

The cost of the optimization is around ~3-5% of compilation speed
with complex shaders.

V2: by Jason Ekstrand
    - Move nir_sweep up, right after the last change of NIR

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=103274


Signed-off-by: Danylo Piliaiev <danylo.piliaiev@globallogic.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Cc: mesa-stable@lists.freedesktop.org
(cherry picked from commit d2195213)