asahi: Serialize NIR in memory

asahi: Serialize NIR in memory

Deserializing isn't expected to be much more expensive than cloning, and the
serialized NIR is *significantly* smaller. So store the serialized instead of
the deserialized, and deserialize on the fly.

This reduces a lot of noise in valgrind due to random crap alloc'd against the
NIR shader by lowering passes that now get properly freed.

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24258>