[mlir][linalg] ComprehensiveBufferize: Do not copy InitTensorOp results

E.g.:

```
%2 = memref.alloc() {alignment = 128 : i64} : memref<256x256xf32>
%3 = memref.alloc() {alignment = 128 : i64} : memref<256x256xf32>

// ... (%3 is not written to)

linalg.copy(%3, %2) : memref<256x256xf32>, memref<256x256xf32>
vector.transfer_write %11, %2[%c0, %c0] {in_bounds = [true, true]} : vector<256x256xf32>, memref<256x256xf32>
```

Avoid copies of %3 if %3 came directly from an InitTensorOp.

Differential Revision: https://reviews.llvm.org/D109742