nir: Add ability to lower non-const quad broadcasts to const ones.

nir: Add ability to lower non-const quad broadcasts to const ones.

Some hardware doesn't support subgroup shuffle, and on such hardware
it makes no sense to lower quad broadcasts to shuffle. Instead, let's
lower them to four const quad broadcasts, paired with bcsel instructions.

Cc: mesa-stable@lists.freedesktop.org
Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4147>