nir: avoid generating conflicting output variables

nir: avoid generating conflicting output variables

Because not all vertex outputs can have corresponding fragment inputs
(eg. edgeflags) some logic is needed to correctly generate variables in
a passthough gs.

Before this change some output variables ened up with the same location.

Fixes: d0342e28b32 ("nir: Add helper to create passthrough GS shader")

Acked-by: Mike Blumenkrantz <michael.blumenkrantz@gmail.com>
Reviewed-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21238>