[mlir] Add loop.parallel, loop.reduce and loop.reduce.return operations.

Summary:
These operations can be used to specify a loop nest with a body that can
contain reductions. The iteration space can be iterated in any order.

RFC: https://groups.google.com/a/tensorflow.org/d/topic/mlir/pwtSgiKFPis/discussion

Differential Revision: https://reviews.llvm.org/D72394