pan/bi: Lower load_output to make sysval explicit

pan/bi: Lower load_output to make sysval explicit

See previous commits for justification. Later, we'll split up NIR processing in
a few steps to give the caller a chance to lower the sysval, at which point the
goofy inputs here will go away.

v2: Only lower in fragment shaders. Likely harmless to run elsewhere but still
wrong because the location enum is defined per-stage.

Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com> [v1]
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20906>