i965/fs: Lower large local arrays to scratch

Shader-db results on Kaby Lake:

    total instructions in shared programs: 15280819 -> 15231557 (-0.32%)
    instructions in affected programs: 72578 -> 23316 (-67.87%)
    helped: 6
    HURT: 2

    total cycles in shared programs: 374751200 -> 348895164 (-6.90%)
    cycles in affected programs: 27979445 -> 2123409 (-92.41%)
    helped: 6
    HURT: 2

    total spills in shared programs: 32215 -> 31807 (-1.27%)
    spills in affected programs: 2784 -> 2376 (-14.66%)
    helped: 6
    HURT: 2

    total fills in shared programs: 37570 -> 25944 (-30.94%)
    fills in affected programs: 11740 -> 114 (-99.03%)
    helped: 6
    HURT: 2

The helped shaders are all compute shaders in Aztec Ruins.  There is
also a compute shader in synmark2 OglCSDof that's helped but it doesn't
show up in above shader-db results because it went from SIMD8 to SIMD16.
That shader improves enough to yield an 15-20% performance boost to the
benchmark as a whole on my KBL laptop.  The two hurt shaders are
everyone's favorite KSP shaders that have a large block of constants.
15 jobs for !360 with review/nir-scratch in 13 minutes and 4 seconds (queued for 1 second)
detached
Status Job ID Name Coverage
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passed #232351
ubuntu

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passed #232352
autotools

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passed #232354
meson-clang

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passed #232360
meson-gallium-clover-llvm

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passed #232361
meson-gallium-clover-llvm39

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passed #232359
meson-gallium-drivers-other

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passed #232358
meson-gallium-radeonsi

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passed #232362
meson-gallium-st-other

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passed #232353
meson-gallium-swr

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passed #232357
meson-glvnd

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passed #232356
meson-loader-classic-dri

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passed #232355
meson-vulkan

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passed #232364
scons-llvm

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passed #232363
scons-nollvm

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passed #232365
scons-swr

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