Do not allocate registers for non-data dependencies

[apt1002]

In mod pressure, we currently allocate exactly one result register for each Node. Instead, each Node should know how many results it has.

• Add a method to enum Op to say how many results it has.
• In Pressure::allocate_register(), put the second half of the function in a loop that repeats for each result (maybe zero times).
• Add division to enum Op. It has two results.

[apt1002]

Fixed by 98ef71ae0903a24c3001364df6c4010a0a260bd6.