``verbatim`` and control flow side effects

Looks like `verbatim` is considered to have defaulted control flow side effects, so it's e.g. assumed not to successfully terminate a transaction.

This should either be changed or at least be clearly documented, since it is well possible to construct `verbatim` that very much does affect control flow behaviour, which would be ill-behaved for example wrt the unused store eliminator.

Start by locating the compiler handling for `verbatim` control-flow side effects and the unused store eliminator mentioned in the issue. Determine how `verbatim` is currently classified and whether the intended outcome is changed behavior or documentation; done means the chosen semantics are implemented or clearly documented, with relevant compiler tests updated.

Written by the indexing model from the issue text.