llvmpipo/nir: free compute shader NIR

I forgot this in the last round.

Fixes: 18f896e55d96 (llvmpipe: add initial nir support)

Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4899>
1 file changed