[mlir][bufferization][NFC] Bufferize with PostOrder traversal

This is useful because the result type of an op can sometimes be inferred from its body (e.g., `scf.if`). This will be utilized in subsequent changes.

Also introduces a new `getBufferType` interface method on BufferizableOpInterface. This method is useful for computing a bufferized block argument type with respect to OpOperand types of the parent op.

Differential Revision: https://reviews.llvm.org/D128420