In addition to treating comparisons with literals as sanitisers, we now
also treat comparisons with variables that have a single assignment as
sanitisers.
Proving that such a variable is actually a constant is not easy, but for
this use case a simple approximation works fine.
This generalises our previous handling of sanitisers operating on property accesses to support dynamic property accesses where the property name is an SSA variable by representing them as access paths.
