[mlir][bufferization] Better propagation of bufferizesToMemoryWrite through regions

`bufferizesToMemoryWrite(OpResult)` looks for OpOperands that bufferize to memory writes inside the region of the defining op (if it has one). Currently, if the reverse use-def chain stops at any value inside of the region, the OpResult is considered to bufferize to a memory write.

It is always safe to have false positives among `bufferizesToMemoryWrite`, so the previous implementation is also correct. However, it can lead to additional buffer copies.

Differential Revision: https://reviews.llvm.org/D142223