[mlir][bufferization] Do not canonicalize to_tensor(to_memref(x))

This is a partial revert of D128615.

to_memref(to_tensor(x)) always be folded to x. But to_tensor(to_memref(x)) cannot be folded in the general case because writes to the intermediary memref may go unnoticed.

Differential Revision: https://reviews.llvm.org/D129354