This works similarly to the CL compiler, where a 64-bit address is
decomposed into a 32-bit index and offset. But unlike CL, where the
index is into a per-kernel array of bound buffers, for Vulkan it points
into the global device-wide descriptor heap.
For all global deref chains that terminate in a load/store/atomic, create
a parallel deref chain that begins by decomposing the pointer to a vec2,
followed by a load_vulkan_descriptor, and then an SSBO deref chain. Any instance
where the original deref chain was used for something else will remain as
global derefs, so also run lower_explicit_io for global to produce appropriate
pointer math.
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/28028>
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