spirv: Implement OpCopyObject as a blind copy

Because the types etc. are required to be identical, we can just copy-propagate the guts of the vtn_value. This is a very partial solution. Ideally, we've got a bit of work to do to make all the composite stuff able to handle all types including images, sampler, and combined image/samplers but this gets some CTS tests passing.

Cc: mesa-stable@lists.freedesktop.org

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