nir: Add nir_alu_src_as_uint helper

nir: Add nir_alu_src_as_uint helper

We have a few ALU instructions that take a constant source. Technically, they
have a swizzle so you can't just nir_src_as_uint them, even though a bunch of
backends do. To help backends do the right thing, add a helper that's just as
easy to use that will chase the swizzle properly.

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/22695>