[mlir] Introduce callback-based builders to SCF Parallel and Reduce ops

Similarly to `scf::ForOp`, introduce additional `function_ref` arguments to
`::build` functions of SCF `ParallelOp` and `ReduceOp`. The provided functions
will be called to construct the body of the respective operations while
constructing the operation itself. Exercise them in LoopUtils.

Differential Revision: https://reviews.llvm.org/D81872