Annotating ssa defs without affecting compilation is impossible with
debug info instructions since referencing a nir_def from the debug info
instr will add uses.
The old approach also stops worrking if passes reorder instructions.
This patch proposes a solution which should not regress performance just
like the old approach. The difference is that this one allocates a bit
more space for debug info instead of adding a new instruction for it.
Reviewed-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/33141>
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