Storelike boilerplate is too strict in dead blocks

[MattWindsor91]

While I've been slowly making progress on the Storelike boilerplate's ability to turn off some of its bookkeeping in dead code blocks, there are still a few things that don't seem to be there yet:

• making sure we don't erase known value records inside dead code, regardless of whether the corresponding flag is set;
• removing some extra destination restrictions (but not all - we need things like 'needs known value' for the generator to work);
• possibly others.

The second one probably needs a new parameter being passed into Basic.extra_dst_restrictions to clarify whether the insertion is happening in dead code or not.