nir/lower_blend: Avoid useless iand with logic ops

nir/lower_blend: Avoid useless iand with logic ops

The upper bits start correctly, there's no need to clear them as long as we keep
them zero'ed by using ixor with a valid bit mask instead of inot.

Makes the code generated for logic op slightly less ridiculous. I'm joking. It's
still ridiculous but I'm not in the mood to fix up the Midgard compiler and it's
just a little ALU for a feature almost nothing uses.

Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Reviewed-by: Gert Wollny <gert.wollny@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20016>