gallivm: Silence a warning at -Og

gallivm: Silence a warning at -Og

   ../src/gallium/auxiliary/gallivm/lp_bld_arit.c: In function ‘lp_build_round_arch’:
   ../src/gallium/auxiliary/gallivm/lp_bld_arit.c:2042:7: warning: ‘intrinsic_root’ may be used uninitialized in this function [-Wmaybe-uninitialized]
    2042 |       lp_format_intrinsic(intrinsic, sizeof intrinsic, intrinsic_root, bld->vec_type);

Can't happen, mark it unreachable.

Reviewed-by: Eric Anholt <eric@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8724>