Handle blocks that have multiple paths to the same successor

Especially with `continue` and `break`, it's very possible to end up in a situation where there are multiple exits from a block to the same block, as:
```meson
foreach a: [b, c, d]
  if meson.has_header(a)
    break
  endif
endforeach
```
will end up in the tail of the loop block if the break statement is reached or if the foreach loop is exhausted.