intel/ibc: Load g0 with a LOAD_PAYLOAD and stuff it in the shader

Merged Faith Ekstrand requested to merge wip/ibc-g0-load-payload into ibc

This should allow CSE to be able to combine uses of g0 so we can CSE texture headers again. It's not a great solution but it seems better than special-casing g0 all over the IR.

Related: #27 (closed)

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