nir: Delete nir_register::uses and nir_register::if_uses

We do piles of book-keeping work to keep track of them but never used
them for anything.

Acked-by: Karol Herbst <kherbst@redhat.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
15 jobs for !588 with wip/nir-all-ssa in 13 minutes and 43 seconds (queued for 2 seconds)
latest detached
Status Job ID Name Coverage
  Containers Build
passed ubuntu #223068

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  Build+Test
passed autotools #223069

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passed meson-clang #223071

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passed meson-gallium-clover-llvm #223077

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passed meson-gallium-clover-llvm39 #223078

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passed meson-gallium-drivers-other #223076

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passed meson-gallium-radeonsi #223075

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passed meson-gallium-st-other #223079

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passed meson-gallium-swr #223070

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passed meson-glvnd #223074

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passed meson-loader-classic-dri #223073

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passed meson-vulkan #223072

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passed scons-llvm #223081

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passed scons-nollvm #223080

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passed scons-swr #223082

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