nir/lower_clip: Only emit 1 discard

nir/lower_clip: Only emit 1 discard

If we have multiple clip planes, rather than emit multiple discards we can just
OR together the discard criteria. Then a nir_opt_algebraic rule kicks in to
optimize out the flt/.../flt/ior/.../ior into fmin/.../fmin/flt, generating
much less code at the end.

Written while debugging an unrelated issue with the clip lowering.

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Reviewed-by: Faith Ekstrand <faith.ekstrand@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21103>