[mlir][bufferize] Fix bug in AllocTensorElimination

AllocTensorElimination does currently not support chains where the type is
changing. AllocTensorElimination used to generate invalid IR for such
inputs. With this commit, AllocTensorElimination does no longer apply to
such inputs. (It can be extended to support such IR if needed.)

Differential Revision: https://reviews.llvm.org/D131880