[mlir][bufferize] Allow non-equivalent yields from scf.for loops

This removes a restriction wrt. scf.for loops during One-Shot Bufferization. Such IR was previously rejected. It is still rejected by default because the bufferized IR could be slow. But such IR can now be bufferized with `allow-return-allocs`.

Differential Revision: https://reviews.llvm.org/D121529