lima/ppir: refactor const lowering

lima/ppir: refactor const lowering

Const nodes are now cloned for each user, i.e. const is guaranteed to have
exactly one successor, so we can use ppir_do_one_node_to_instr() and
drop insert_to_each_succ_instr()

Tested-by: Andreas Baierl <ichgeh@imkreisrum.de>
Reviewed-by: Qiang Yu <yuq825@gmail.com>
Reviewed-by: Erico Nunes <nunes.erico@gmail.com>
Signed-off-by: Vasily Khoruzhick <anarsoul@gmail.com>