glsl: fix incorrect optimisation in opt_constant_variable()

glsl: fix incorrect optimisation in opt_constant_variable()

When handling function inputs the optimisation pass incorrectly
assumes the inputs are undefined. Here we simply change things to
assume inputs have always been assigned a value. Any further
optimisations will be taken care of once function inlining takes
place.

Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/2984
Fixes: 65122e9e8038 ("ir_constant_variable: New pass to mark constant-assigned variables constant.")
Reviewed-by: Danylo Piliaiev <danylo.piliaiev@globallogic.com>
Reviewed-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5413>