i965/link: Serialize program to nir after linking for shader cache

i965/link: Serialize program to nir after linking for shader cache

If the shader cache is enabled, after linking the program, we
serialize the program to nir. This will be saved out by the glsl
shader cache support.

Later, if the same program is found in the cache, we can use the nir
for a fallback in the unlikely case that the gen binary program is not
found in the cache.

Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>