[MLIR][OpenMP] Allow the single operation to have a region

The OpenMP standard specifies the region inside to be a structured
block. This means that branches into or out of the region is not
permitted but branches are permitted inside the OpenMP single region.
This is currently modeled by having a region of kind `AnyRegion` for
all the OpenMP ops. In future we can have a more strictly defined
region.

Fixes #58396.

Reviewed By: peixin

Differential Revision: https://reviews.llvm.org/D137023