CT: pc_on_data_is_ignored fails when filtering "is"

[facuMH]

originally found by @simonlechner. Running this rule on its own runs fine, but if ran filtering by "is" produce a failure.
This could be because on the current conversion from CT state to Geth state there is no way of informing geth about which segments of the code are data or not. Another possibility could be that the code generator has a dependency on certain value of the rng (this has been an issue in the past).

This rule should be kept around since it adds to the completeness of the specification (providing a rule for all the random states that can be generated where PC is on data), but the effect should be worked so that we do not impose any behaviour on the resulting state.

This task requires more investigation that should be carried at a more convenient time.

[simonlechner]

The failed CT run can be reproduced by running go run ./go/ct/driver/ run -f _is_ -full-mode geth.