This patch updates the handling of target regions to set trip counts and
kernel execution modes properly, based on clang's behavior. This fixes a
race condition on `target teams distribute` constructs with no `parallel
do` loop inside.
This is how kernels are classified, after changes introduced in this
patch:
```f90
! Exec mode: SPMD.
! Trip count: Set.
!$omp target teams distribute parallel do
do i=...
end do
! Exec mode: Generic-SPMD.
! Trip count: Set (outer loop).
!$omp target teams distribute
do i=...
!$omp parallel do private(idx, y)
do j=...
end do
end do
! Exec mode: Generic-SPMD.
! Trip count: Set (outer loop).
!$omp target teams distribute
do i=...
!$omp parallel
...
!$omp end parallel
end do
! Exec mode: Generic.
! Trip count: Set.
!$omp target teams distribute
do i=...
end do
! Exec mode: SPMD.
! Trip count: Not set.
!$omp target parallel do
do i=...
end do
! Exec mode: Generic.
! Trip count: Not set.
!$omp target
...
!$omp end target
```
For the split `target teams distribute + parallel do` case, clang
produces a Generic kernel which gets promoted to Generic-SPMD by the
openmp-opt pass. We can't currently replicate that behavior in flang
because our codegen for these constructs results in the introduction of
calls to the `kmpc_distribute_static_loop` family of functions, instead
of `kmpc_distribute_static_init`, which currently prevent promotion of
the kernel to Generic-SPMD.
For the time being, instead of relying on the openmp-opt pass, we look
at the MLIR representation to find the Generic-SPMD pattern and directly
tag the kernel as such during codegen. This is what we were already
doing, but incorrectly matching other kinds of kernels as such in the
process.
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