Walk the whole vertex stride thanks to XFB info sorted by offset, gather
individual components from same or different outputs, and once we have
gathered 4, store them as vec4.
It also removes the COHERENT flag from VMEM stores because NGG streamout
doesn't use it either and I don't think it's needed.
Reviewed-by: Timur Kristóf <timur.kristof@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/32686>
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