Handle blocks that have multiple paths to the same successor

[dcbaker]

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:

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.