[flang] legacy branch target (#75628)

Branching to an endif statement from outside of the if is nonconformant:

  subroutine jump(n)
    goto 6
    if (n == 3) then
      goto 7
  6 end if
    print *, 'pass'
    return
  7 print *, 'fail'
  end

However, this branch was permitted up to f90. Account for this usage
when rewriting if constructs and if statements by suppressing rewriting
if the end statement is labeled.