anv: Insert holes for non-existant XFB varyings

Thanks to optimizations, it's possible for varyings to get deleted but
still leave the variable there for nir_gather_xfb_info to find.  If we
get into this case, insert a hole.

Fixes: 36ee2fd6 "anv: Implement the basic form of..."
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Tested-by: Marge Bot <mesa/mesa!3520>
Part-of: <mesa/mesa!3520>
39 jobs for !3520 with review/anv-xfb-dce-fix in 29 minutes and 15 seconds (queued for 4 seconds)
detached
Status Name Job ID Coverage
  Container
passed arm_build #1474797
aarch64

00:00:26

passed arm_test #1474799
aarch64

00:00:26

passed x86_build #1474792

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passed x86_build_old #1474796

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passed x86_test-gl #1474794

00:00:19

 
  Build
passed meson-arm64 #1474806
aarch64

00:11:39

passed meson-armhf #1474805
aarch64

00:11:04

passed meson-clang #1474807

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passed meson-clover #1475025

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passed meson-clover-old-llvm #1474811

00:06:33

passed meson-i386 #1474813

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passed meson-main #1474803

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passed meson-mingw32-x86_64 #1474814

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passed meson-testing #1474801

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passed meson-vulkan #1474812

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passed scons #1474815

00:12:07

passed scons-old-llvm #1474816

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passed scons-swr #1474808

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passed scons-win64 #1474809

00:04:34

failed meson-clover #1474810

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  Test
passed arm64_a306_gles2 #1474835
db410c

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passed arm64_a630_gles2 #1474832
mesa-cheza

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passed arm64_a630_gles3 #1474834
mesa-cheza

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passed arm64_a630_gles31 #1474833
mesa-cheza

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passed lima-mali450-test:arm64 #1474820
lava-meson-gxl-s905x-libretech-cc

00:09:47

passed panfrost-t720-test:arm64 #1474817
lava-sun50i-h6-pine-h64

00:09:02

passed panfrost-t760-test:armhf #1474818
lava-rk3288-veyron-jaq

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passed panfrost-t860-test:arm64 #1474819
lava-rk3399-gru-kevin

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passed piglit-glslparser #1474822

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passed piglit-quick_gl #1474821

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passed piglit-quick_shader #1474823

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passed test-llvmpipe-gles2 #1474824

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passed test-softpipe-gles2 #1474825

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passed test-softpipe-gles31 1/4 #1474828

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passed test-softpipe-gles3 1/2 #1474826

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passed test-softpipe-gles31 2/4 #1474829

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passed test-softpipe-gles31 3/4 #1474830

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passed test-softpipe-gles31 4/4 #1474831

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passed test-softpipe-gles3 2/2 #1474827

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