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pan/va: Mark last register reads

Alyssa Rosenzweig requested to merge alyssa/mesa:bi/last-reads into main 
On Valhall, register reads may be marked as "last" [1]. Setting the last flag
promises the hardware that the value of the register is no longer required. This
may enable hardware optimizations. In particular, it may permit the hardware to
avoid register file writes if a write to the marked register is still in the
forwarding buffer. This may improve power efficiency.

In principle, this is trivial: run liveness analysis and mark killed sources,
like we would in an SSA-based register allocator. In practice, there are a few
wrinkles to avoid hazards around staging registers and 64-bit register pairs,
requiring some additional data flow analysis and fix ups. However, nothing here
is particularly "hard", and all the ideas are already in use for the Bifrost
scheduler and the Bifrost/Valhall scoreboard analyses.

[1] In Mesa's compiler, this is called discard for historical reasons.

(+ supporting patches and some clean up.)

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