st: Sweep NIR after linking phase to free held memory

st: 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.

Signed-off-by: Danylo Piliaiev <danylo.piliaiev@globallogic.com>
Reviewed-by: Eric Anholt <eric@anholt.net>