glsl/nir: Fix copying 64-bit values in uniform storage

The iterator `i` already walks the right amount now that is
incremented by `dmul`, so no need to `* 2`.  Fixes invalid memory
access in upcoming ARB_gl_spirv tests.

Failure bisected by Arcady Goldmints-Orlov.

Fixes: b019fe8a "glsl/nir: Fix handling of 64-bit values in uniform storage"
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
10 jobs for !1162 with r/glsl-nir-columns-typo in 7 minutes and 48 seconds (queued for 1 second)
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Status Job ID Name Coverage
  Containers Build
passed #378903
debian

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  Build+Test
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meson-clang

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meson-clover

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meson-main

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meson-swr-glvnd

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meson-vulkan

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scons-llvm

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scons-nollvm

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scons-swr

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scons-win64

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