This turned out to be a fairly simple but easy to make bug. When we want to
figure out the value pointed-to in a multi-assignment, we look at the left hand
side to see what value from the right hand side we should assign. Unfortunately,
we accidentally attempted to look up this information in the _left hand side_ of
the assignment, resulting in no points-to information at all. The only thing
needed to fix this was to properly link up the left and right hand sides: using
the left hand side to figure out what index to look at, and then looking up the
points-to information for the corresponding place in the right hand side.
