nir: add a bool bitsize lowering pass

The pass lowers 1-bit booleans produced by NIR to the native bitsize
of the operations that produce them.

v2: change on lower_load_const_instr after upstream changes. Added
    TODO2 to explain it, as it was not properly tested yet (see
    already existing TODO) (Neil)
13 jobs for !1044 with mediump-nir in 12 minutes and 1 second (queued for 4 seconds)
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Status Job ID Name Coverage
  Containers Build
passed #485607
debian

00:00:18

 
  Build+Test
passed #485616
meson-arm64

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passed #485615
meson-armhf

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passed #485609
meson-clang

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passed #485613
meson-clover

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passed #485617
meson-i386

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passed #485612
meson-main

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passed #485608
meson-swr-glvnd

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passed #485614
meson-vulkan

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passed #485619
scons-llvm

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passed #485618
scons-nollvm

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passed #485610
scons-swr

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passed #485611
scons-win64

00:11:40