[mlir][bufferize] Eliminate tensor.empty ops instead of bufferization.alloc_tensor ops

tensor.empty op elimination is an optimization that brings IR in a more bufferization-friendly form. E.g.:

```
%0 = tensor.empty()
%1 = linalg.fill(%cst, %0) {inplace = [true]}
%2 = tensor.insert_slice %1 into %t[10][20][1]
```

Is rewritten to:

```
%0 = tensor.extract_slice %t[10][20][1]
%1 = linalg.fill(%cst, %0) {inplace = [true]}
%2 = tensor.insert_slice %1 into %t[10][20][1]
```

This optimization used to operate on bufferization.alloc_tensor ops. This is not correct because the documentation of bufferization.alloc_tensor says that it always bufferizes to an allocation. Instead, this optimization should operate on tensor.empty ops, which can then be lowered to bufferization.alloc_tensor ops (if they don't get eliminated).

Differential Revision: https://reviews.llvm.org/D137162