[mlir][bufferization] Decouple buffer-deallocation from One-Shot Bufferize

The buffer deallocation pass must now be run explicitly when `allow-return-alloc` is set.

This results in a few extra buffer copies in unoptimized test cases. The proper way to avoid such copies is to relax the OpOperand/OpResult aliasing contract on ops such as scf.for. Some of these copies can also be avoided by improving the buffer deallocation pass.

Differential Revision: https://reviews.llvm.org/D126252