User varyings are linked by both name and register. The name is based
on how many *variables* are before it in final driver_location sort
order, not necessarily how many registers are before it.
In some cases where clip/cull distance are involved, it's possible
for one shader to write into a part of the cull distance that's
ignored by a downstream shader, but because linking is done by
*whole* register locations, and clip/cull can be combined using
*fractional* register locations, this is hard to detect. Since no
non-sysvals end up using fractional locations, just put all non-sysvals
first so they always generate the same semantic names for the same
register locations.
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20346>