Collapse two nested ifs into a single multiway if

This PR is not trying to simplify the whole nested structure,
just this one layer, as an incremental simplification.

It relies on:

if a:
  x
else:
  if b:
    y
  else:
    z

being the same as:

if a:
  x
elif b:
  y
else:
  z

Author: benclifford