Partial data flow had a semantic merge conflict with this branch. The
problem is that partial data flow doesn't (and shouldn't) cause the
initial pruning steps to run, but the length-2 access paths depend on
the `consCand` information that comes from that initial pruning. The
solution is to restore the old `AccessPath` class, now called
`PartialAccessPath` for use only by partial data flow.
With this change, partial data flow will in some cases allow more field
flow than non-partial data flow.
