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: #2984 (closed) Fixes: 65122e9e ("ir_constant_variable: New pass to mark constant-assigned variables constant.")