[mlir][linalg][bufferize] Bufferize using PostOrder traversal

This is required for bufferization of scf::IfOp, which is added in a subsequent commit.

Some ops (scf::ForOp, TiledLoopOp) require PreOrder traversal to make sure that bbArgs are mapped before bufferizing the loop body.

Differential Revision: https://reviews.llvm.org/D111924