[MLIR][Linalg] Rename `packElementWiseOp` to `packGenericOp` (NFC)

Commit b4563ee1 enabled propagation for
pack and unpack through non-elementwise operations, update comments and
methods names to reflect the changes made.

Rework some tests where the `linalg.generic` was reading from
`tensor.empty`, which is undefined behaviour.

Reviewed By: hanchung, qedawkins

Differential Revision: https://reviews.llvm.org/D149952