--2c--stack: fix a bug in code generated for array concat.

Now all the tests that were ok in the heap mode are also ok in the
stack mode !

However the code generated by fillred and co is weird.

For instance, in should_work.pipeline.lus, in
fillred_oneStep_pipe_10_step, accu_in is used both as an input and an
output, which seems wrong in the stack mode as no copy is performed.

Is it working by chance ?