lima/ppir: support pipeline registers in scheduler

lima/ppir: support pipeline registers in scheduler

The ppir scheduler grew to be rather complicated and containing many
exceptions as it also has to take care of inserting additional nodes
when it is mandatory for nodes to be in the same instruction.
As such, the lima lowering and scheduling process can be difficult to
understand and maintain.
The ppir lowering step created nodes hoping that the scheduler would
notice the exception and do the right thing.

This proposal adds a simple refactor to the scheduler so that it places
nodes with pipeline registers in the same instruction.
With the scheduler handling this in a general way, it is possible to
create same-instruction dependencies by using pipeline registers during
the lowering stage.
This is simpler to maintain because now we can make these dependencies
explicit in a single place (lowering), and we can drop exceptions from
scheduling.
Reducing the complexity of the scheduler is also useful as preparatory
work to support control flow in ppir.

Signed-off-by: Erico Nunes <nunes.erico@gmail.com>
Reviewed-by: Vasily Khoruzhick <anarsoul@gmail.com>
Reviewed-by: Qiang Yu <yuq825@gmail.com>