[mlir][bufferization] Support unstructured control flow

This revision adds support for unstructured control flow to the bufferization infrastructure. In particular: regions with multiple blocks, `cf.br`, `cf.cond_br`.

Two helper templates are added to `BufferizableOpInterface.h`, which can be implemented by ops that supported unstructured control flow in their regions (e.g., `func.func`) and ops that branch to another block (e.g., `cf.br`).

A block signature is always bufferized together with the op that owns the block.

Differential Revision: https://reviews.llvm.org/D158094