[MLIR][OpenMP] Add omp.simd operation

This patch introduces the `omp.simd` operation. In contrast to the existing
`omp.simdloop` operation, it is intended to hold SIMD information within
worksharing loops, rather than representing a SIMD-only loop. Some examples
of such loops are "omp do/for simd", "omp distribute simd", "omp target teams
distribute parallel do/for simd", etc. For more context on this work, refer to
PR #79559.

This operation must always be nested within an `omp.wsloop` operation as its
only non-terminator child. It follows the same approach as the `omp.distribute`
operation, by serving as a simple wrapper operation holding clause information.