[m-rev.] for post-commit review: factor out mode simplification code

[Julien Fischer]

On Sun, 15 Nov 2020, Zoltan Somogyi wrote:

> For review by anyone.
>

> Factor out common code for simplifying modes.
> 
> We used to have several predicates whose job was to simplify modes for
> presentation to users. Their jobs were slightly different, but contained
> a shared core: recognizing the exploded (from_inst >> to_inst)
> forms of the standard builtin modes. Replace these

That's fine.

Julien.