[mlir][bufferization] Allow to_memref ops in One-Shot Analysis

bufferization.to_memref ops are allowed in One-Shot Bufferize, but they are treated conservatively: in the absence of a memref analysis, we have to assume that the result buffer is read and written.

Note: to_memref cannot introduce any future aliases that would have to be considered during One-Shot Bufferize, because only to_tensor ops with the `restrict` attribute are supported. Such tensors are guaranteed to not alias with any other buffer after bufferization.

Differential Revision: https://reviews.llvm.org/D153365