glsl: remove dead varyings before assigning attr locations

glsl: remove dead varyings before assigning attr locations

The GLSL IR code used to clean these up. Some apps such as the Heaven
benchmark fail linking if we don't eliminate them before the linker
validation.

Fixes: a8534c7b4cfc ("glsl: drop the dce of global vars from GLSL IR linker")
Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/9319

Tested-by: Marcus Seyfarth <m.seyfarth@gmail.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24035>