nir/algebraic: Optimize integer cast-of-cast

These have been popping up more and more with the OpenCL work and other
bits causing extra conversions to/from 64-bit.

Reviewed-by: Karol Herbst <kherbst@redhat.com>
14 jobs for !714 with review/nir-opt-convert in 13 minutes and 4 seconds (queued for 2 seconds)
detached
Status Job ID Name Coverage
  Containers Build
passed debian #263871

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  Build+Test
passed meson-clang #263873

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passed meson-gallium-clover-llvm #263879

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passed meson-gallium-clover-llvm39 #263880

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passed meson-gallium-drivers-other #263878

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passed meson-gallium-radeonsi #263877

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passed meson-gallium-st-other #263881

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passed meson-gallium-swr #263872

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passed meson-glvnd #263876

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passed meson-loader-classic-dri #263875

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passed meson-vulkan #263874

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passed scons-llvm #263883

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passed scons-nollvm #263882

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passed scons-swr #263884

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