[mlir][bufferize] Do not deallocate allocs that are returned from a block

Such IR is rejected by default, but can be allowed with `allow-return-memref`. In preparation of future refactorings, do not deallocate such buffers.

One-Shot Analysis now gathers information about yielded tensors, so that we know during the actual bufferization whether a newly allocated buffer should be deallocated again. (Otherwise, it will leak. This will be addressed in a subsequent commit that also makes `allow-return-memref` a non-experimental flag.)

As a cleanup, `allow-return-memref` is now part of OneShotBufferizationOptions. (It was previously ignored by AlwaysCopyBufferizationState.) Moreover, AlwaysCopyBufferizationState now asserts that `create-deallocs` is deactivated to prevent surprising behavior.

Differential Revision: https://reviews.llvm.org/D121521

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