[mlir][linalg][bufferize][NFC] Simplify InsertSliceOp bufferization

No need to keep track of equivalent extract_slice / insert_slice tensors during bufferization. Just emit a copy, it will fold away.

Note: The analysis still keeps track of equivalent tensors to make the correct inplace bufferization decisions.

Differential Revision: https://reviews.llvm.org/D116684