[mlir] Extended BufferPlacement to support nested region control flow.

Summary: The current BufferPlacement implementation does not support
nested region control flow. This CL adds support for nested regions via
the RegionBranchOpInterface and the detection of branch-like
(ReturnLike) terminators inside nested regions.

Differential Revision: https://reviews.llvm.org/D81926

Author: dfki-mako