ac/nir: Fix store_scratch with a non-full writemask

By adding one more helper to ac_llvm_build, we can also easily keep
vector stores together.

Fixes the
tests/spec/glsl-1.30/execution/fs-large-local-array-vec4.shader_test
piglit test.

Fixes: 74470bae ("ac/nir: Lower large indirect variables to scratch")
Reviewed-by: Marek Olšák <marek.olsak@amd.com
17 jobs for !1700 with ac-nir-scratch-fix in 13 minutes and 53 seconds (queued for 2 seconds)
latest detached
Status Name Job ID Coverage
  Containers Build
passed debian #512541

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  Build+Test
passed meson-arm64 #512550

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passed meson-armhf #512549

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passed meson-clang #512543

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passed meson-clover #512547

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passed meson-i386 #512551

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passed meson-main #512546

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passed meson-swr-glvnd #512542

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passed meson-vulkan #512548

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passed scons-llvm #512553

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passed scons-nollvm #512552

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passed scons-swr #512544

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passed scons-win64 #512545

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  Test
passed test-llvmpipe-gles2 1/4 #512554

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passed test-llvmpipe-gles2 2/4 #512555

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passed test-llvmpipe-gles2 3/4 #512556

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passed test-llvmpipe-gles2 4/4 #512557

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