Add an option to not check dead code

[wmdietl]

a61489c disables not checking dead code.
Up to now, the philosophy was to check all code, as it might become un-dead at some later point.
Think about adding an option to not check dead code.

There also seems to be bad interaction with the Initialization Checker. This might be related to #619 (comment); maybe for some checker the dead code detection gives the wrong results.

[wmdietl]

For an example why ignoring dead code is bad, see this fix: eisop-codespecs/daikon@c4f838f
The debug branch is dead code, which then leads to this true error to be missed. When debugging is enabled, that printf might crash. One would need to be careful to type-check the code again when debugging is enabled, which one might not do.