i965/anv: use nir_opt_vectorize_io()

Commit 8d822246 caused substantially more URB messages in
geometry and tessellation shaders (due to enabling
nir_lower_io_to_scalar_early). This combines io again to avoid
this regression while still allowing link time optimisation of
components.

Shader-db results (SKL):

total instructions in shared programs: 13109035 -> 13107191 (-0.01%)
instructions in affected programs: 66278 -> 64434 (-2.78%)
helped: 242
HURT: 13

total cycles in shared programs: 332090418 -> 332094364 (<.01%)
cycles in affected programs: 285477 -> 289423 (1.38%)
helped: 39
HURT: 215

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=107510
24 jobs for vectorize_io_v2 in 76 minutes and 11 seconds (queued for 1 minute and 30 seconds)
latest
Status Job ID Name Coverage
  Build+Test
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build:make-gallium-drivers-other

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build:make-gallium-drivers-radeonsi

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build:make-gallium-st-clover-llvm-39

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build:make-gallium-st-clover-llvm-4

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build:make-gallium-st-clover-llvm-5

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build:make-gallium-st-clover-llvm-6

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build:make-gallium-st-clover-llvm-7

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build:make-gallium-st-other

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build:make-loader-classic-dri

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build:make-vulkan

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build:meson-gallium-clover-llvm5

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build:meson-gallium-clover-llvm6

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build:meson-gallium-clover-llvm7

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build:meson-gallium-drivers-other

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build:meson-gallium-radeonsi

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build:meson-gallium-st-other

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build:meson-gallium-swr

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build:meson-glvnd

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build:meson-vulkan

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build:scons-llvm

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build:scons-nollvm

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build:scons-swr

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