iris: Serialize the NIR to a blob we can use for shader cache purposes.

iris: Serialize the NIR to a blob we can use for shader cache purposes.

We will use a hash of the serialized NIR together with brw_prog_*_key
(for NOS) as the disk cache key, where the disk cache contains actual
assembly shaders.

Reviewed-by: Dylan Baker <dylan@pnwbakers.com>