This predicate computed a local CP between all defs and uses of the same
virtual variable in a basic block. This wasn't a problem in
`unaliased_ssa`, but it became a huge problem in `aliased_ssa`, probably
because many variables can be modelled with a single virtual variable
there.
Before this commit, evaluation of `aliased_ssa`'s
`variableLiveOnEntryToBlock#ff#antijoin_rhs` on Wireshark took 80
_minutes_. After this commit, that predicate and its immediate
dependencies take around 5 _seconds_.
4ac22253eb