Rather than open-coding indices with manual bit packing flying around,
let's add a data structure corresponding to a reference to some data.
(Think nir_src, ibc_ref, etc). In particular this allows us to pack in
more metadata, like an offset, for properly supporting limited vectors
(for I/O) without bloating the IR with swizzle fields.
Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8215>
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