[flang][OpenMP] Assertion failure lowering `!$omp tile` with an unstructured loop body

Lowering an `!$omp tile` construct whose associated loop body is unstructured
fails an assertion in `createEmptyRegionBlocks`:

```
flang/include/flang/Lower/DirectivesCommon.h:57:
void Fortran::lower::createEmptyRegionBlocks(fir::FirOpBuilder &,
std::list &)
[TerminatorOps = ]:
Assertion `mlir::isa(terminatorOp) && "expected terminator op"' failed.
```

## Reproducer

`tile_unstructured.f90`:

```fortran
subroutine tile_unstructured(x, y, n)
implicit none
integer n, i, j
double precision x(*), y(*)
!$omp tile sizes(2,2)
do i = 1, n
do j = 1, n
if (x(i) > 0.0d0) then
y(1) = 0.0d0
cycle
end if
y(2) = 1.0d0
end do
end do
end subroutine
```

```
flang -fopenmp -fopenmp-version=51 -c tile_unstructured.f90
```

Start with the reproducer in tile_unstructured.f90 and run flang -fopenmp -fopenmp-version=51 -c tile_unstructured.f90. Then inspect createEmptyRegionBlocks in flang/include/flang/Lower/DirectivesCommon.h to trace the assertion during lowering of the unstructured loop body. Done means the reproducer no longer triggers the assertion.

Written by the indexing model from the issue text.