Currently we use a heuristic to avoid marking the exit of the last
basic block, expecting it to be a `return` and so that execution
will cease whether we evaluate it or not. However, it is possible
to write code for which the final statement is a `GotoNode` and
in this case we can get incorrect answers if we fail to evaluate it.

This example illustrates a tricky point: `return` both terminates
execution but may also return a value. If we don't require the
value, we still might need to terminate execution. This example
seems to illustrate that having `isrequired[i]` be either `true` or
`false` may be insufficiently expressive; we might need it to be
three states, `:no`, `:yes`, `:exit`. During marking, encountering
`:exit` would not force one to evaluate the returned SSAValue.