nir: Accept nir_var_mem_global in derefs used by phis

This mode is used by PhysicalStorageBufferEXT storage class.

Fixes: 8bdf5a00 "nir: Allow derefs to be used as phi sources"
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
10 jobs for !976 with r/nir-validate-global in 7 minutes and 53 seconds (queued for 2 seconds)
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passed debian #327510

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passed meson-clang #327512

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passed meson-swr-glvnd #327511

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passed scons-llvm #327519

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passed scons-swr #327513

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passed scons-win64 #327514

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