nir: Fix constant folding for irhadd/urhadd

nir: Fix constant folding for irhadd/urhadd

This should be a subtract, not an add. The comment's proof is correct,
but the (wrong) expression we actually use isn't what it's in the
comment! Correct the discrepancy.

The lowering in nir_opt_algebraic was correctly typed.

Fixes: 272e927d0e9 ("nir/spirv: initial handling of OpenCL.std extension opcodes")
Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Karol Herbst <kherbst@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/11671>