pan/bi: Dead code eliminate per-channel

pan/bi: Dead code eliminate per-channel

We already track the full liveness so this is a trivial optimization,
with an especial win for shaders reading only a subset of components of
gl_FragCoord.

More importantly, it's required for proper scheduling (in soft mode)
when vectors are used and some (but not all components) are promoted to
temporary registers.

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8354>