[flang][OpenACC] stop statement in one construct inside one acc region

```f90
subroutine test_stop_in_region3()
  integer :: x
  !$acc parallel
    x = 3
    if (x > 1) stop x
  !$acc end parallel
end
```
```console
$ bbc -emit-fir -fopenacc test.f90 
error: loc("/home/qpx/compilers/llvm-community/example/fix-exit-region/acc/test.f90":4:5): operation with block successors must terminate its parent block
FATAL: verification of lowering to FIR failed
```