[MLIR][Linalg] Change destination logic in `bubbleUpPackOpThroughGenericOp`.

In `bubbleUpPackOpThroughGenericOp`, we replaced the init operands with
a new `tensor.empty` if the operation was a pure element-wise op. This
behaviour is not wrong but not ideal because we "break" the original
use-def-chain of the output operand by materializing a new
`tensor.empty`. We should use `tensor.empty` as a destination *only* if the
initial init operand was already a `tensor.empty`, as we do in
`PushDownUnpack`.

Reviewed By: hanchung

Differential Revision: https://reviews.llvm.org/D149250