nir_to_tgsi: fix shared memory index

The index for shared memory should be set to TGSI_MEMORY_TYPE_SHARED
which matches the index used in the declaration.

Fixes spec@arb_compute_shader@execution@shared-atomic* with SVGA driver
193 jobs for !16458 with fix-ntt-shared-mem-index in 41 minutes and 13 seconds (queued for 15 seconds)
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