Allow forward jumps to "unreachable" labels

[erkyrath]

We now permit this pattern:

if (condition) jump MiddleLabel;
if (0) {
    while (condition) {
      ...
    .MiddleLabel;
      ...
    }
}

This is similar to the example in the release notes, except that the jump is before MiddleLabel. So when we reach MiddleLabel, we know it's in use.

This pattern, weird as it looks, is regularly used by Inform 7 adjective code. Inciting example:

To tally (D - a description of things): say "Tallied."

When play begins:
        tally all open doors.

[erkyrath]

This doesn't change behavior for any code that compiled under 6.40. It just compiles more code.

Test case: https://github.com/erkyrath/Inform6-Testing/blob/forward-jumps/src/branchprune-fwd.inf