[MLIR][OpenMP] Skip host omp ops when compiling for the target device (#85239)

This patch separates the lowering dispatch for host and target devices.
For the target device, if the current operation is not a top-level
operation (e.g. omp.target) or is inside a target device code region it
will be ignored, since it belongs to the host code.


This is an alternative approach to #84611, the new test in this PR was
taken from there.