in sccp, when the operand of `jnz` or `assert` is already an IRLiteral
(this is most easily seen by disabling the IRnode branch eliminator),
the compiler will panic.

refactor: rename _get_lattice to _from_lattice, change it to a method

* changed the index type of lattice to `IRVariable` (it should only
  have variables)
* add getter/setter methods that also check types to cache future
  regretions
* add `_eval_lattice_with_op()` to evaluate the lattice with an operant
* revert `_eval()` changes
* unify the handling of `store`

This reverts commit 8010d7f.

This reverts commit 2aecae3.

This reverts commit 4c03199.