[mlir] Extend Operation visitor with pre-order traversal

This patch extends the Region, Block and Operation visitors to also support pre-order walks.
We introduce a new template argument that dictates the walk order (only pre-order and
post-order are supported for now). The default order for Regions, Blocks and Operations is
post-order. Mixed orders (e.g., Region/Block pre-order + Operation post-order) could easily
be implemented, as shown in NumberOfExecutions.cpp.

Reviewed By: rriddle, frgossen, bondhugula

Differential Revision: https://reviews.llvm.org/D97217