iris: Serialize a stripped copy of the NIR and only save the sha1

Kenneth Graunke requested to merge kwg/mesa:iris-sha-only into master

Space and time improvements suggested by Jason on IRC earlier today.

Instead of stripping the NIR of debugging information, serializing it, and saving the whole blob of serialized IR in the iris_uncompiled_shader, we now work on a copy, and only save the sha.

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