[mlir] Refactor code out of BufferPlacement.cpp

Now BufferPlacement.cpp doesn't depend on Bufferize.h.

Part of the refactor discussed in:
https://llvm.discourse.group/t/what-is-the-strategy-for-tensor-memref-conversion-bufferization/1938/17

Differential Revision: https://reviews.llvm.org/D89268