nir: Avoid coalescing vars created by lower_io_to_temporaries

nir: Avoid coalescing vars created by lower_io_to_temporaries

Right now nir_copy_prop_vars is effectively undoing
nir_lower_io_to_temporaries for inputs by propagating the original
variable through the copy created in lower_io_to_temporaries. A
theoretical variable coalescing pass would have the same issue with
output variables, although that doesn't exist yet. To fix this, add a
new bit to nir_variable, and disable copy propagation when it's set.

This doesn't seem to affect any drivers now, probably since since no one
uses lower_io_to_temporaries for inputs as well as copy_prop_vars, but
it will fix radv once we flip on lower_io_to_temporaries for fs inputs.

Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>