[MLIR][OpenMP] Verify loop wrapper properties of omp.parallel (#88722)

This patch extends verification of the `omp.parallel` operation to check
it is correctly defined when taking a loop wrapper role.

In OpenMP, a PARALLEL construct can be either a (potenially combined)
block construct or a loop construct, when appearing as part of a
composite construct. This is currently the case for the DISTRIBUTE
PARALLEL DO/FOR and DISTRIBUTE PARALLEL DO/FOR SIMD exclusively.

When used to represent the PARALLEL leaf of a composite construct, it
must follow the rules of a wrapper loop operation in MLIR, and this is
what this patch ensures. No additional restrictions are introduced for
PARALLEL block constructs.