glsl/nir: free GLSL IR right after we convert to NIR

glsl/nir: free GLSL IR right after we convert to NIR

Gives us memory back faster which is useful for pathalogical CTS
tests.

The GLSL IR was previously used after converting to NIR for things
like building the GL resource list but we have had a NIR version
for this for some time and I don't believe there are any other
use cases left for keeping the old IR hanging around this long.

Reviewed-by: Emma Anholt <emma@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15127>